That’s the point, only with god can you be certain. Otherwise I could say the universe popped into being last Thursday and you came with all the memories you have, despite how ludicrous it sounds you can’t be certain it’s not true.
To know something you have to know everything. I know I have hair because I can see it. I can see it because I own a mirror. I have a mirror because I remember buying it etc etc. To know A you have to know b, to know B you have to know C and this doesn’t stop at Z it is infinite.
whonhas infinite knowledge? God