It's not in the Constitution. It's in a letter from Jefferson to the Danbury Baptist Church.
https://www.loc.gov/loc/lcib/9806/danpre.html

Today that phrase, "separation of church and state" is used to dismiss Christianity. In an attempt to weaken the Constitution. This doesn't have to be a "Christian" nation for individuals with an understanding of scripture to serve in politics. It is where morality is found. Which is why it is under such heavy attack today.