I'm not the one saying he doesn't exists. You are. That's like saying the onus is on me to prove I exist or you exist.
Atheists go around saying he doesn't. They can't and don't prove it. They won't admit to doubt on the issue, so the onus is on atheist to tell us poor, intellectually challenged rubes why you adhere to this adamant surety, this doubt-free certitude of man as supreme being.
Let me make it easy for you: If there is no God, then atheists imply that man is the supreme being. They should prove it or admit doubt.
You are trying to use semantic flim flam to avoid answering the question. Why no doubt?
It's a little different. If I say "I have two sons" and you don't believe me, I can prove their existence by showing them to you, showing their birth certificates etc. It is incredibly difficult, if not impossible, for you to prove that they do not exist.
Same thing with God. You say he exists. You're the one who can prove his existence. I have no way to disprove His existence.