those were the beliefs of our forfathers
this nation was founded on christian beliefs, not jewish, not muslim, not athiest. christian. the christians who founded this nation also believed in freedom of religion, show me any other religious group throughout the world who has established that sort of freedom... not saying "hail christianity", only that the founders of the constitution had the best of intentions.
also take into account the time period it was written in. this was the time of the quakers and the shakers. people who actually did not believe in sex. at all... not even for procreation. religion was the most prevelent part of their lives, so why not include it in their legislation. however no where in the constitution does it impose christianity on anyone. it mearly says "Almighty God" which like stated before refers to any/all Gods you choose to worship.
also take into account that the Americans did not wish to alienate themselves from the rest of the world. all other major nations were still ruled by christian leaders, and influenced by the pope. Had america decided to rule out any religious talk in the constitution, it would have seemed as though they were an athiest nation, and that would NOT have created any allies, which was necessary in their weakened state.