Sorry but it is impossible for you, or anyone else, to say they are 100% certain that there is no God, afterlife etc just as it is impossible for anyone to 100% say there is. None of us know.
It is impossible to be 100% certain we are not all figments of some giant computer's simulation of reality.
It is impossible to be 100% certain the stars are not holes in a big sheet of black velvet just out of our reach.
It is impossible to be 100% certain that the world isn't being ruled by 10 foot tall alien lizards with a penchant for human flesh (though opinion differs as to whether they prefer us raw, or lightly toasted with some sort of piquant sauce.)
None of these are likely but it is impossible to be 100% certain!
So, the question then becomes a question of crediblity.
Billions of years of random chemistry accidentally knocking together some self-replicating molecules (or even just one as it's self- replicating), or a sentient creator (origin unknown) who/which for (unknown reason) decides to create (by inexplicable means) a sentient being and then play Sims with it?