The First Amendment. It's quite simple really.<quoted text>
Read the constitution and please tell me where it says separation of church and state. IT DOESN'T
And what's more, a bunch of cranky Christians requested clarification as to what it means, hence Jefferson wrote a letter explaining it in detail, which is where the original term "separation of church and state" comes from.
Of course you already know this, what being an American who knows about American history and all, right?