Hi U U, interesting question!
I have no idea about the religious side of it, but if I were to interpret it linguistically, I would say that it is a kind of metaphor to mean that even though a word has a fixed root, for example, trust, because 'its branches are in the sky,' it can be changed with something like an affix: mistrust, trustful, trustworthy etc. So while the root is always fixed, the rest of the word can change.