America is not a "Christian nation". America is a secular, constitutional republic founded on Christian principles. Yes, there is a difference and that difference is huge.
A Christian nation would be a theocratic state, fascist in intent, that demanded conformity to Christianity.
A secular, constitutional republic founded on Christian principles is a cooperative of individuals, based on law, where each individual is free to believe and act in ways they deem right provided they do not violate the common law.
The First Amendment protects both freedom of speech and freedom of religion. It does not demand conformity to Christian speech, Christian values, or the Christian religion. Nor does it demand freedom from religious dialogue and influence regardless of whether that dialogue and influence is Christian, Muslim, Buddhist, Animist, Mystic, Shamanist, Atheist, or anything else imaginable. It was written that way intentionally.
Naturally, Shari'a is just as incompatible with the Constitution as Leviticus would be.
You and I have every right to speak our mind. We have no right to demand laws forcing everyone else to agree.
That is what "freedom" really means.