The Constitution was not written during one of our nation’s spasms of religious fervor, and most of the Founding Fathers were nominal Christians at best. Nevertheless, it was an era in which Christianity was taken seriously, and disputes between religious groups were a matter of considerable public importance. The large number of sects in this country made it completely impracticable to create a state religion. Hence, the Establishment Clause.
For over two centuries, it was assumed that the best way to avoid taking sides in religious disputes was to keep the government out of the religion business altogether. Establishment Clause jurisprudence reflects that assumption. Today’s Supreme Court pretty clearly does not accept that premise, however. Why not?
The Supreme Court appears to view the fundamental disagreement on religion in contemporary society as one between secular humanism on one hand and all types of religion on the other, not one between religious sects. I think you are going to see the Court increasingly making the argument that funding religious sects is acceptable because the alternative is making secular humanism (whatever that means) a kind of established religion. In other words, times and thought patterns have changed, so the interpretation of the Establishment Clause has to change, too.
The irony, of course, is that originalists absolutely reject this ahistorical approach when it works to the benefit of the cultural left. For the reactionaries on the Court, however, consistency is the hobgoblin of little minds.