There can be no unalienable set of rights without it being endowed by something outside of itself. Man can certainly come to the realization that his rights are unalienable but he must reason that this unalienability has to be endowed by something outside of mankind in order to remain unalienable by its very definition. This something, as the Founders rightly, whole-heartedly agreed with, is God.
Well existence and the law of identity are outside of man. That is why I talked about an objective moral code. A god given code of morality is by definition a subjective code of morality.