So while Vesca is discussing about Kupala, I'm going to share my view about inventing Gods.
I believe that Deities can be born out of "human worship" alone. I believe that a community can change the image of an original Deity and create an entirely new Deity through sheer force of TRUST and FAITH over an extended period of time. I believe that there are Gods and Goddesses of the Old that are continuously being created anew, in all the otherworlds, where thoughts, actions, and experiences give rise to Deities of different kinds. Every version of worship or praise that we do I believe is an act of transformation in the spiritual world, which is sacred in itself. And this aspect of concentrated spirituality is the chief component of that which becomes a new Deity.
More precisely, the person who performs his own system of beliefs, creates own rules and standards on how to reach out to the higher worlds, develops own perception about how his God or Goddess looks like, and gives names to his Supreme Being according to his belief, in so doing, invents his own Deity or make an older Deity evolve or transform. Such Deity, however, connected in its essence to the man who created it, still lives, on the whole, in a different dimension of being. And it's in the otherworld that faith acquires substance, and, in turn, influences the worlds above.
This is the reason why I don't believe that the Christian God is the same deity as Yahweh (YHVH or YHWH) who is the God of the Jews, I do believe that He evolved from the earlier Yahweh and now exists as a Deity in His own right whose name is simply "God" or "Jehovah", exactly as the Christians view him.