You can certainly think you know that God exists.
But you can also think you know that God doesn't exist.
In both cases you could be wrong.
How do we know when we actually know something? When it comes to God - we suddenly find ourselves in a philosophical labyrinth of madness. Let us bring to mind Descartes' evil genius construct. A suitably evil and powerful genius could convince us that we know God, when in fact we don't - and vice versa.
For example:
You wake up - you are sure you have just been in a terrible car crash. God stands over you (or however you conceive God in this circumstance), and introduces you to the residents of heaven, reveals some great secret allows you live in paradise for what seems like a thousand years. Then he sends you back to earth. Naturally - you know God exists, you met him personally!
Of course, whose to say that a wicked spirit didn't kidnap you, erase your memory, planted a memory of a car crash and then forced you to have a religious hallucination. I therefore call on the 'brain in a jar' thought experiment to demonstrate that we can never really be a 1 or a 7 since we can never know anything to a confidence level of 100%
If we accept the possibility that supernatural agents could exist - we have to accept the possibility that a supernatural agent exists who is not God, who could do the above. This entity could convince you that God has ceased to exist - thus you'd know that God does not exist (anymore).
You could be certain God exists, have utter faith that it is the case that He does. Still - you cannot philosophically have no doubt whatsoever. If you cannot admit to some level of doubt about something you are either not thinking on the right level, or you are suffering from a delusion.
I am not even a '1' on that scale when it comes to the existence of the keyboard I'm typing on. Is there any revelation that I can have that would mean I would know God better than I know this keyboard? How would someone differentiate that revelation from delusion? How would I know from my own frame of reference that I wasn't deluded when God revealed Himself? Indeed - an evil genius could simply remove the doubt from my mind using supernatural powers.
If somebody knows that they are Napolean, they know that Napolean is alive. Of course, they don't know they are Napolean - they are deluded into thinking with 100% certainty that they are Napolean and their delusion allows no room for doubt.
That's the philosophy side of things, as for the logic? Logically speaking: anything
P1: I say that I know God does not exist
P2: What I say I know, I know
C: I know God does not exist
Assuming no evil geniuses or delusions
1, you can know that God exists by meeting Him. However, you can know that God does not exist by meeting Him and then Him telling you that he is going to cease to exist, followed by Him ceasing to exist.
1Gigantic and unwarranted assumption
™ Version 4.2
