Volokh examines it:

Like many eloquent generalities, the phrase “wall of separation between church and state” doesn’t really tell us much about the concrete disputes that we face today. It can’t be taken literallysurely, for instance, the fire department may cross the “wall” of a church to put out a fire, and the police may go to the church to investigate a crime (or even arrest people who are being sheltered in the church). And once one steps away from the literal meaning, one sees a great deal of ambiguity.

Is it a breach of the wall of separation for the government to give vouchers to parents for their children’s education, whether the vouchers are redeemed at a religious school or a secular school? I think it isn’t, just as there’s no breach of “separation” when the government provide other benefits evenhandedly to all institutions or all students without regard to religionbenefits such as trash pickup, nonprofit tax exemptions, or college student loans. But others disagree with me on that. Is it a breach of the wall of separation for the government to use religious speech in its official pronouncements (without giving any special funds or rights to any particular church), as Jefferson himself did in the Declaration of Independence and in the Virginia Bill for Religious Freedom? People disagree on that, too.

It’s clear that Jefferson opposed a union of church and state along the English lines, in which there was one established Anglican church whose bishops automatically sat in Parliament, and whose members had special rights that members of other churches did not have. And on this most Americans likely agreed with him (though on other questions of religion Jefferson was probably not that representative of public sentiment). But while “separation” clearly meant a prohibition on such a union, and would have been broadly endorsed by Americans in that sense, the metaphor of the “wall of separation” doesn’t really help us understand how we should resolve concrete church-state questions today, or how early Americans would have resolved those questions.

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