The founders felt religion should not be part of the government.  

"I believe in one God, and no more; and I hope for happiness beyond this life.

I believe in the equality of man; and I believe that religious duties consist in doing justice, loving mercy, and endeavoring to make our fellow-creatures happy.

But, lest it should be supposed that I believe in many other things in addition to these, I shall, in the progress of this work, declare the things I do not believe, and my reasons for not believing them.

I do not believe in the creed professed by the Jewish church, by the Roman church, by the Greek church, by the Turkish church, by the Protestant church, nor by any church that I know of. My own mind is my own church.

All national institutions of churches, whether Jewish, Christian or Turkish, appear to me no other than human inventions, set up to terrify and enslave mankind, and monopolize power and profit."

- Thomas Paine - Age of Reason