Jefferson adds religious language into the Declaration of Independence at the beginning and the end, generally as a way to justify the declaration by implying that God's on the side of the colonists. People are entitled to equality by "Nature's God" (1), and "endowed by their Creator with certain unalienable Rights" (2).

At the end, when he actually gets around to declaring independence, the author appeals "to the Supreme Judge of the world for the rectitude of our intentions" (46), and claims to be supported by a "firm reliance on the protection of divine Providence" (47).

At this time in American and European history, religion (almost exclusively some form of Christianity) played a significant role in almost everyone's life. The way that Jefferson uses references to God and religion indicates that those references are meant to add gravitas and support not only the delegates' claims to independence, but Jefferson's loftier philosophical claims as well.

Jefferson is sure that he and his compatriots have His support, and that could go a long way among people for whom Christianity was a major part of their lives. People would happily argue with you, but they wouldn't argue with the Big Guy upstairs.