Man, you're in deep.
So, the clerks who wrote the final draft of the US Constitution included "A.D". Like you say, everyone included A.D. at the time, because that was the thing to do, it was done by Christians without thinking.
That is not an endorsement of Christianity.
That is not a declaration of Christianity as the official religion of the United States.
That is not an endorsement of God.
And it certainly isn't an endorsement of a particular branch of Protestant Christianity that didn't even exist at the time.