The Establishment clause of the First Amendment, did not require the separation of church and state. Establishment is the support of a specific religion with public funds. At the time of adoption, a number of states, most notably Massachusetts, has established religions. The separation of church and state is a good idea, but it wasn't written into the constitution.
By the time of the 14th amendment, no states had established religions, but many religious practices were incorporated into laws. It wasn't until 1947 that the Supreme Court talked about the separation of church and state and not until the 1960's that school prayer was struck down. A good discussion of the history can be found at
Establishment Clause.
Not all the founding fathers were Christian. Thomas Paine was an Atheist. Thomas Jefferson was a Deist. Only John Adams was actually religious.
A few years ago, I asked a friend who was a part-time minister about Trump having violated all 10 commandments. He said it didn't matter because "Trump's policies were good for the poor". We joked with him that his next sermon should about the "10 suggestions".