The court has held that student-led prayers in a football huddle constitute an establishment of religion. Ditto an invocation offered by a rabbi at a public high school graduation. There, Justice Kennedy explained that asking non-believers to stand and "maintain a respectful silence" was unconstitutional. Respectful silence just isn't the spirit of the age.
The state of Texas urges the Court to adopt the reasonable person standard for evaluating the Ten Commandments monument. Would a reasonable person, seeing this granite slab, assume that Texas meant to enforce a ban on graven images or to force neighbors to refrain from covetousness? The brief did jocularly offer that "no one would reasonably think that the state has adopted a position, one way or the other, on whether the Dallas Cowboys should continue playing professional football on Sundays or whether the Texas Longhorns should continue playing college football on Saturdays (notwithstanding the seriousness, and even religious fervor, with which Texans approach their football ...)."
The real point is that we've lost our grip on any common-sense definition of establishment. The Founders did not want to favor one church over another at the federal level (when the Constitution was ratified, several states did have established churches). By forbidding one national church pre-eminence, freedom of worship would be more reliably protected. The notion that this country, founded firmly in the Judeo-Christian tradition, could not even mention God in public without fearing a subpoena is simply ludicrous.
If the Supreme Court hands down a ruling that the Texas monument violates the Constitution, it will do so in the literal shadow of a frieze on the Supreme Court's chamber depicting none other than Moses holding the tablets in his hands.