What reasons would you seperate Church and State?

I have an essay on it. Basically, 75% of this country is some form of Christian. Now, I’m not arguing for an atheist nation but one that is more religiously open. So that if another country is talking about us, they can’t say, “Oh, the Cristian country right?”

So, my 3 big claims are that:
-It would reduce tension with various nations and religious factions (Taliban)
-It would make immigration easier for those of various religions
-It was reduce (but not eliminate) debate about things such as gay marriage, abortion, etc….

Would you add any other strong reasons? And if I offend anyone with this post I do apologize.