In reply to your question, I guess that depends on the proof you provide, is it physical? Scientific? Esoteric?
Do you believe in magic?
Maybe the first proof that can be made was
(logic) Topic: Computer Scientists Prove God Exists Ideas aren't reality
The theorem says that God, or a supreme being, is that for which no greater can be conceived. God exists in the understanding. If God exists in the understanding, we could imagine Him to be greater by existing in reality. Therefore, God must exist.
The theorem says that my unicorn is that for which nothing loving me more can be conceived. My unicorn exists in the understanding. If my unicorn exists in the understanding, we could imagine Her to be loving me even more if she actually knows me. Therefore, my unicorn must exist.
Kthxbai