If A is bigger than B, and B is bigger than C, then A is bigger than C - we know that
But, when it comes to empirical facts (aside from pure logic), no, we can't 'know' anything 100%.
The technical definition of knowledge still isn't agreed upon. But, for the most part, it's having a justified belief in something that is TRUE (plus some other property to avoid Gettier problems - basically, to avoid being 'lucky').
So, you can 'know' that God exists if you are justified in believing in him AND he really exists. Ironically, we don't have to 'know' that he exists in order to 'know' that he exists provided that you're justified AND he exists (even if we can't prove it)...lol@philosophy.