The reality of clerical magic is not per se evidence of the existence of the gods.
I was reading The Kobold Guide to Worldbuilding and one of the articles was discussing the difficulty of atheism or agnosticism in a world where clerics have real magical powers. I think nothing could be further from the truth.
While most clerics (or even most people) might believe that their powers come from a divine source that does not foreclose other possibilities. Perhaps clerics have gained powers through the brain altering power of prayer and meditation. Perhaps it is a form of sorcery that is simply not well understood but ascribed to higher powers. Or perhaps the "gods" are in fact other beings masquerading as gods.
Just because someone says that something is the cause of another does not make it so.
Perhaps wizards loath clerics for their dogmatic and irrational beliefs.
Perhaps all wizards are traditionally elves and all clerics are traditionally humans and it is mere racial magic passed down along with the spiritual traditions of the race.
Perhaps things can be mysterious and ambiguous!