JUSTICE GROMETER delivered the opinion of the court:

Plaintiff, the Du Page County Board of Review (Board), appealsa judgment affirming a decision by the Department of Revenue of theState of Illinois (Department) granting a tax exemption for realproperty owned by Good Shepherd Evangelical Lutheran Church (GoodShepherd or the church). The Board contends that the Departmenterred insofar as it held that a house for one of the church'sschoolteachers is exempt under section 15--40 of the Property TaxCode (35 ILCS 200/15--40 (West 1998)). We agree. Therefore, weaffirm in part and reverse in part.

On May 11, 1999, Good Shepherd, a member of the WisconsinEvangelical Lutheran Synod, bought land improved with a detachedgarage and a house. The land is near the church's other property. On August 13, 1999, the church petitioned the Department to exemptits new property from taxation for 66% of the 1999 tax year. TheBoard contested the petition. We summarize the evidence from thehearing before the administrative law judge (ALJ).

Good Shepherd's first witness was Erik Guldberg, the presidentof its congregation. He testified as follows. When thecongregation wishes to "call" a new teacher, it obtains a list ofcandidates from the synod. The voters' assembly then chooses acandidate, and the church sends her a "call letter." On March 8,1999, the church sent a letter to Ellen Zank, "solemnly charg[ing]"her to instruct her students in "the chief truths of the Word ofGod[]" and otherwise to follow Christian principles. Also, theletter requires Zank to assist as the school's athletic directorand to conduct the vacation Bible school. In return, the churchpromises to receive Zank as "a servant of Jesus Christ," to treather accordingly, and (more mundanely) to pay her salary and provideher housing. On May 10, 1999, the congregation voted to requireZank to live in the house.

David Rutschow, Good Shepherd's senior pastor, testified thatthe "ministry" includes both the church and the school. He statedthat "we're calling [pastors and teachers] to the ministry of ourcongregation, whether it be preaching or whether it be teachingministry." A synod yearbook for 2001 lists Zank among the "Womenin the Teaching Ministry."

A letter Rutschow wrote his attorney in August 2000 explainsthat the church calls pastors to the "preaching ministry" andteachers to the "teaching ministry." The teachers must instructstudents in the standard secular subjects and in the Bible.Teachers must teach all subjects and administer all discipline inthe light of the Bible.

The synod operates Martin Luther College in Minnesota, whichtrains pastors and teachers, both of whom are "presented to thechurch as candidates for the ministry" and receive their initialassignments from the synod. Teachers and pastors are called by themethod Guldberg described.

Rutschow recounted that, on August 8, 1999, Zank was installedduring a church service. The installation was conducted accordingto a form entitled "A Rite of Installation/Ordination of Teachers." Zank professed her faith in scripture and promised that herteaching would follow the appropriate religious doctrines.

Ellen Zank testified as follows. At Martin Luther College,she received religious education but no pastoral training. Shearrived at Good Shepherd a month before her installation. Zankteaches kindergarten through second grade. Good Shepherd'steachers teach "all the subjects that you would find in any otherschool" and do so "in the light of God's word." Zank teachesspecifically religious subjects an hour each day. She is theathletic director and helps with vacation Bible school.

Zank uses one of the two bedrooms in her house as a "schooloffice," where she grades papers and does teaching-related work. However, the house is never used for meetings or other schoolactivities. Unlike Zank, the principal and the other teachersreside in private homes.

The Board called David Rutschow. Asked why only Zank lives inchurch-owned housing, Rutschow replied that the other teachersalready had their residences and that "we had a single teacherpreviously who left us through death ***---she had been housed inan apartment because she was a single teacher." The congregationhad paid this teacher's rent. Were Zank not living in the church-owned house, the congregation would give her a reasonable housingallowance. There were nearby apartment houses that thecongregation believed were safe and well run.

The ALJ recommended exempting the property at issue undersection 15--40, which at the pertinent time read:

"All property used exclusively for religious purposes, or usedexclusively for school and religious purposes, or fororphanages and not leased or otherwise used with a view toprofit, is exempt, including all such property owned bychurches or religious institutions or denominations and usedin conjunction therewith as housing facilities provided forministers *** performing the duties of their vocation asministers at such churches or religious institutions or forsuch religious denominations ***.

A parsonage, convent or monastery or other housingfacility shall be considered under this Section to beexclusively used for religious purposes when the church ***requires that the above listed persons who perform religiousrelated activities shall, as a condition of their employmentor association, reside in the facility." 35 ILCS 200/15--40(West 1998).

The ALJ concluded that the house is used exclusively for areligious purpose because (1) Zank's "call" requires her to livethere; and (2) Zank performs many of her job duties there. The ALJdid not decide whether the house is exempt as a housing facilityfor a "minister." The ALJ also held that the detached garage isexempt because it is reasonably necessary for the church to storechurch-related property there.

The Department adopted the ALJ's recommendation. The circuitcourt affirmed, although on the ground that the house was theresidence of a "minister." The Board appeals.

The Board does not contest the exemption for the detachedgarage. However, it claims that Zank's home is not tax-exempt. The Board argues that because the facts are undisputed, theexemption issue is a question of law to be reviewed de novo. According to the Board, the Department erred as a matter of law inconcluding that the house is "used exclusively for religiouspurposes" (35 ILCS 200/15--40 (West 1998)), as the house isprimarily a residence. The Department responds that even thoughthe facts are not in dispute, the Department's decision should notbe reversed unless it is clearly erroneous--which, of course, inthe Department's view, it is not.

We first address whether the Department's decision should bereviewed de novo or deferentially. The Board's assertion that thisappeal raises an issue of law, to be reviewed de novo, is inherently plausible. Generally, whether a given set of historicalfacts satisfies a given constitutional or statutory standard isconsidered a question of law subject to de novo review. See, e.g.,People v. Sims, 192 Ill. 2d 592, 615 (2000) (whether given factssupply probable cause); People v. Lamborn, 185 Ill. 2d 585, 590(1999) (whether given photograph meets statutory definition of"lewd"); In re Marriage of Hughes, 322 Ill. App. 3d 815, 818-19(2001) (whether facts reflect "substantial change of circumstances"under child-support statute). Indeed, such was long the rule inproperty tax exemption cases. In 1996, our supreme court stated,"Where facts are undisputed, *** whether property is exempt fromtaxation is a question of law." Chicago Patrolmen's Ass'n v.Department of Revenue, 171 Ill. 2d 263, 271 (1996).

However, we agree with the Department that the supreme courthas rewritten the rules for reviewing administrative agencies'decisions and has overruled the cited passage from ChicagoPatrolmen's Ass'n. See Carpetland U.S.A., Inc. v. IllinoisDepartment of Employment Security, 201 Ill. 2d 351, 368-69 (2002). Under the new dispensation, whether given historical facts satisfyan established legal rule is a "mixed question of law and fact,"and an agency's resolution of that question must stand unless it is"clearly erroneous." AFM Messenger Service, Inc. v. Department ofEmployment Security, 198 Ill. 2d 380, 392-93 (2001); City ofBelvidere v. Illinois State Labor Relations Board, 181 Ill. 2d 191,205 (1998). This rule of deference rests in part on agencies'experience and expertise in interpreting their governing statutes. AFM, 198 Ill. 2d at 394-95. The "clearly erroneous" test appliesto Department rulings in property tax exemption cases. Swank v.Department of Revenue, 336 Ill. App. 3d 851, 860-61 (2003).

What the "clearly erroneous" test actually means in thiscontext--what it requires a court of review to do--is perhaps ill-defined. Positing the existence of a "continuum" between de novoreview of issues of law and deferential "manifest weight" review offactual issues, the supreme court has called "clearly erroneous" an"intermediate standard of review" providing "somewhat lessdeference" to the agency than that given on purely factualquestions. Carpetland U.S.A., 201 Ill. 2d at 369. Yet the court has said that a decision is "clearly erroneous" only if we are " 'left with the definite and firm conviction that a mistake hasbeen committed.'[Citation.]" AFM, 198 Ill. 2d at 395. Thissuggests that the test is essentially reducible to reasonablenessand is thus, for all practical purposes, as deferential as the"manifest weight" test used for purely factual issues.

Additionally, the court has modeled the "clearly erroneous"test on the similar federal standard for reviewing purely factualfindings. AFM, 198 Ill. 2d at 393-95. Yet the federal standard isextremely deferential. To be "clearly erroneous," a circuitcourt's finding must be " 'more than just maybe or probably wrong;it must *** strike us as wrong with the force of a five-week old,unrefrigerated dead fish.' " Fisher v. Roe, 263 F.3d 906, 912 (9thCir. 2001), quoting Parts & Electric Motors, Inc. v. SterlingElectric, Inc., 866 F.2d 228, 233 (7th Cir. 1988). Thus, themeaning of "clearly erroneous" appears to be elusive, evenconsidering that we cannot expect mathematical precision fromverbal formulae.

Furthermore, it is not clear what practical difference the newtest actually makes. It appears that in Carpetland, AFM, andBelvidere, the court did what it had done before under the de novostandard--apply its precedents to the facts at hand to decidewhether the agency's decision followed the pertinent statutes andcase law. If traditional de novo review is the only way that areviewing court can ascertain whether a mistake has been committed,then nothing has really changed. However, AFM and Belvidere mayaim to give the courts the last word on general questions of lawwhile allowing an agency to "legislate" in a particular case wherejudicial precedent does not dictate the outcome.⁽¹⁾ This followsfrom the rule that agencies' interpretations of their governingstatutes deserve some deference. See AFM, 198 Ill. 2d at 394-95.

Fortunately, we can decide this case without resolving thesetheoretical and practical difficulties. Whatever the "clearlyerroneous" test means, it cannot mean that an agency may floutestablished case law. We believe that the Department's decision isinconsistent with established case law. Thus, it must be reversed.

Under the first paragraph of section 15--40, Zank's house istax-exempt if (1) it is "used exclusively for religious purposes"or (2) "used exclusively for school and religious purposes." 35ILCS 200/15--40 (West 1998). The Department ruled that the houseis "exclusively used for religious purposes." We must infer thatthe Department also implicitly held that the house is used"exclusively for school and religious purposes," as Zank uses partof the house as an office where she grades papers and performsother teaching duties.

Under the second paragraph of section 15--40, the house isexempt if Zank lives there as a condition of her employment (whichshe does) and she is one of the "above listed persons" who performreligion-related activities. 35 ILCS 200/15--40 (West 1998). The"above listed persons" include ministers. Curiously, although therecord is replete with references to Zank's membership in the"teaching ministry" and evidence that her teaching duties includereligious instruction, the Department did not decide whether she isa "minister" under section 15--40. Although the trial court did sorule, that is irrelevant because we review the decision of theagency, not the judgment of the trial court. See Calabrese v.Chicago Park District, 294 Ill. App. 3d 1055, 1065 (1998). Becausethe Department did not rule on this possible ground for a taxexemption, we shall not decide the issue.

We are left with one issue: whether the house is exclusivelyused for religious purposes or school and religious purposes. Wehold that it is not.

The five-room house is primarily a place for Zank to live. Only one room, the bedroom that serves as Zank's office, is usedfor school or religious purposes. No school or religious functionstake place on the property. Although Zank's contract (her call)requires her to live in the house, the evidence does not establishthat the nature of her duties requires her to reside there. Theother teachers and the principal live in private housing. ReverendRutschow's testimony does suggest obliquely that the church mayhave bought the house out of concern for Zank's safety. However,this testimony is unclear at best, and the church never argued thatit felt such a need to provide church-owned housing for Zank. Also, Rutschow's testimony equally suggests that Zank could havebeen housed in reasonably safe and appropriate private lodgings.

Under these circumstances, this case cannot be distinguishedfrom others in which courts have denied tax exemptions. In St.John Evangelical Lutheran Congregation v. Board of Appeals, 357Ill. 69 (1934), the plaintiff church operated a parochial schooland sought a tax exemption for a residence occupied rent-free byone of the school's teachers. The supreme court held that theresidence was not used exclusively for religious purposes orexclusively for school and religious purposes (see Ill. Rev. Stat.1933, ch. 120, par. 2) even though students occasionally receivedtutoring there. Although the house was used partly for school orreligious purposes, its primary use was as a residence. Thus, itwas analogous to parsonages, which, at that time, were not tax-exempt. St. John, 357 Ill. at 71.

In People ex rel. Kelly v. Avery Coonley School, 12 Ill. 2d113 (1957), a private school sought tax exemptions for twoapartment buildings, one of which housed its headmistress andprincipal custodian and the other of which was a residence forunmarried teachers. The school argued that the first buildinghelped the headmistress and the custodian carry out their functionsand saved the school money, while the second alleviated theshortage of suitable housing for unmarried teachers. The courtheld that neither building was used exclusively for school purposes(see Ill. Rev. Stat. 1955, ch. 120, par. 500) because "the primaryuse of the property, and not its incidental uses, has fixed itsstatus for taxing purposes. Here, as in the cases cited, theresidential use is primary; other uses are incidental." AveryCoonley School, 12 Ill. 2d at 116.

In MacMurray College v. Wright, 38 Ill. 2d 272 (1967), twocolleges sought tax exemptions for housing for faculty and staffmembers. The supreme court denied the exemptions, holding that the residences were not used exclusively for school purposes (see Ill.Rev. Stat. 1965, ch. 120, par. 500.1). The property was not exemptmerely because it was school-owned and school personnel livedthere; the schools had to prove that the housing was used primarilyfor purposes that were reasonably necessary to carry out theschools' purposes. However, there was no showing that havingfaculty and staff live in the residences was reasonably necessaryto carrying out their educational duties or that they performedthese duties there. Thus, the residential use of the property wasprimary; the educational use was secondary. As a result, theproperty was not exempt. MacMurray College, 38 Ill. 2d at 278-79.

This case is indistinguishable from the earlier cases. As inSt John, Avery Coonley School, and MacMurray College, the propertyat issue is used primarily as a residence and only secondarily forschool or religious purposes. Although Zank's employment agreementrequires her to live in the house, her job duties do not make itreasonably necessary for her to do so. Other teachers live inprivate housing, and the church could have made the samearrangement for Zank. It was not reasonably necessary to thechurch's function to provide Zank with church-owned housing. Therefore, the house is not tax exempt.

We affirm the grant of a tax exemption for the detachedgarage. We reverse the grant of a tax exemption for the rest ofthe property at issue.

The judgment of the circuit court of Du Page County isaffirmed in part and reversed in part.

Affirmed in part and reversed in part.

BOWMAN and BYRNE, JJ., concur.

1. Thus, the "legislative" function of the agency vis-a-vis thecourts could resemble the legislative function of the courts vis-a-vis the legislature, i.e., the agencies "do and must legislate, butthey can do so only interstitially; they are confined from molar tomolecular motions." Southern Pacific Co. v. Jensen, 244 U.S. 205,221, 61 L. Ed. 1086, 1100, 37 S. Ct. 524, 531 (1917) (Holmes, J.,dissenting).