This is not correct. This decision considered three specific tests; it did not establish any single three-pronged test. Future cases might consider different tests and/or eliminate one or more of the tests used in this case from consideration.
I just wanted to point this out. Wording matters.
LEMON v. KURTZMAN
Three such tests may be gleaned from our cases. First, the statute must have a secular legislative purpose; second, its principal or primary effect must be one that neither advances nor inhibits religion, Board of Education v. Allen, 392 U. S. 236, 243 (1968); finally, the statute must not foster "an excessive government entanglement with religion." Walz, supra, at 674.
Otherwise, I fully acknowledge that these considerations form a vital and necessary threshhold for religious freedom and we should take them very seriously. Having said that, you need to show that the mere act of defining religion fails any of these considerations. On its face, the defining of religion is entirely secular in purpose, neither advances nor inhibits any religion, and does not foster any sort of government entanglement. It's just a definition.