The Constitution derives its power from....the Constitution. The power is organic, derived from the consent of the governed as expressed in regular republican and democratic manner.
Our inalienable rights are not a gift from God- small need of a government if that were the case!- but are created by our having formed a government to define and protect them. This is not only the theory but also the fact of the matter.
None of this is particularly opaque- if you've actually read the document. If not, make some time and do so- it is probably the single most important document in all of history.