Cedric:
Hey!
You have already noted that God has not given conclusive proof of His existence. That is, He has not, for the last 2000 years, seen fit to stand before us and say, "Here I am!" Instead, He has been content simply to leave us evidence of His being. Some folk consider this evidence and reject it while others find the evidence for God's existence compelling. The atheists on this board would have you believe that those who are positively affected by this evidence are ignorant, afraid, deluded, and stupid. But they are not. They are generally as thoughtful, and informed, and sincere in their quest for the truth as any atheist may claim to be. So, what makes people who are equal in most respects, and who are exposed to the same evidence for God's existence, react so differently? Why, God Himself!
The Bible states that God draws us to Him. Were He not to take this initiative, we would never come to a saving faith in Him. And He not only draws us to Him, He keeps us in relationship with Him. So, to answer your question: I am sure God exists because He has given me an inward certainty of His being, that, while supported by convincing, objective evidence, transcends it by touching my heart and not just my mind. God has given me an "inner witness" of His existence that I cannot shake, that comforts me, that informs, secures, and stabilizes my faith in Him. This witness goes deeper than mere words and arguments; it places me in a relationship with God that is personal and powerful and undeniable. I am not left with just words and arguments to bolster my faith in God; I have a daily, personal experience of Him. This is, more than anything else, why I am sure God exists.
This isn't what atheists like to hear, however. They can't argue the minutiae of my personal experience. So, they just dismiss it as delusion or an intellectual cop-out. But, this is to be expected from those who don't know God.
In Christ, Aiki.