I know for certain because of my experience, not because of anything I have read or been told. When I opened my heart to God 6 1/2 years ago, his Spirit came into me in a way that let me know beyond any doubt that he is present with me and in me.
I had believed in God for a while before this happened, but there is a huge difference between believing and knowing. Imagine jumping out of an airplane with a parachute… I could believe all the way to the ground that the parachute would open and allow me to descend safely if I pulled the ripcord. Believing that wouldn’t help much when I hit the ground. But if I actually pulled the ripcord and the parachute deployed and slowed my descent as I believed it would, I would know beyond doubt that it worked.
A lot of people believe in God and just sort of dabble in that belief, mixing it with all sorts of other beliefs. After a while, they begin thinking that belief is useless because they it seems like God isn’t there for them. But if they would have been willing to “pull the ripcord” and trust in God 100%, they would have actually experienced God rather than just dabbling with thinking about him. If they did that, they would know as certainly as I do that God exists and loves them so deeply that he would allow Jesus to die for them so that he could send his Spirit into them.
For me, the Bible or sermons or teaching could not prove God beyond all doubt. But my willingness to place my trust in God 100% and experiencing his presence in me provided that proof, which then validated the Bible for me as the word of God.
I’m more than willing to expand on this answer…just let me know if you want me to.

All caught up on my formspring questions! Time for some new ones… Ask me anything.