I have a feeling that any answer to this question based on law would be found to be unsatisfactory. So there is no point in quoting Jefferson’s Letter to the Danbury Baptists (1802):
Believing with you that religion is a matter which lies solely between Man & his God, that he owes account to none other for his faith or his worship, that the legitimate powers of government reach actions only, & not opinions, I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should “make no law respecting an establishment of religion, or prohibiting the free exercise thereof,” thus building a wall of separation between Church & State.
Similarly, the plain language of the First Amendment plainly reads “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof” If our Founding Fathers had meant for “freedom of religion” to mean only Christianity should exist in America, I’m pretty sure that would have been specified. Likewise, statements of historical fact like quoting the words of Treaty of Tripoli (realizing in law treaties stand equal to the Constitution in precedential value): “As the Government of the United States of America is not, in any sense, founded on the Christian religion..” will likely also not persuade.
So instead why not go to the religious person’s playground to show him/her how wrong s/he is if she thinks religion should be involved with government. In the most widely quoted summary of the relationship between Christianity, secular government, and society, hostile questioners tried to trap Jesus into taking an explicit and dangerous stand on whether Jews should or should not pay taxes to the Roman authorities. The accounts in Matthew 22:15-22 and Mark 12:13-17 say that the questioners were Pharisees and Herodians, while Luke 20:20-26 says only that they were “spies” sent by “teachers of the law and the chief priests”. They anticipated that Jesus would oppose the tax, as their purpose was “to hand him over to the power and authority of the governor”.[Luke 20:20] The governor was Pilate, and he was the man responsible for the collecting of taxes in Roman Judea. At first the questioners flattered Jesus by praising his integrity, impartiality, and devotion to truth. Then they asked him whether or not it is right for Jews to pay the taxes demanded by Caesar. In the Gospel of Mark[12:15] the additional, provocative question is asked, “Should we pay or shouldn’t we?”
Jesus first called them hypocrites, and then asked one of them to produce a Roman coin that would be suitable for paying Caesar’s tax. One of them showed him a Roman coin, and he asked them whose head and inscription were on it. They answered, “Caesar’s,” and he responded: “Render therefore unto Caesar the things which are Caesar’s; and unto God the things that are God’s”. The questioners were impressed. Matthew 22:22 states that they “marveled” (ἐθαύμασαν) and being satisfied with the answer, they went away.
Clearly Jesus was teaching what was to become in America the doctrine of separation of church and state. The state should stay out of religion and religion should stay out of matters of state. The priest needs to heed the words spoke by Jesus himself. Doesn’t Jesus respond to Pontius Pilate about the nature of his kingdom: “My kingdom is not of this world. If my kingdom were of this world, my servants would have been fighting, that I might not be delivered over to the Jews. But now (or ‘as it is’) my kingdom is not from the world” (John 18:36); i.e., his religious teachings were separate from earthly political activity. This reflects a traditional division in Christian thought by which state and church have separate spheres of influence. A more simple explanation is that quite literally the people were not yet of his kingdom for if they were, the servants would rise up against the obvious injustice for convicting an innocent man.