I am getting into this discussion late but here goes.
There are two answers to the question:
The first is that I know that God exists. He/she exists in the hearts and minds of those who believe in him/her/it. The idea of God fills a deep-seated psychological need of a mortal yet thinking creature coping with an apparently infinite universe. God exists, whether as an idea, a yearning or need, call it what you will. Whether this is a good or bad thing is open to debate. There is obviously a need on some psychic level for religion if only evidenced by the amount and variety of it across the world and throughout history.
What would it take for me to believe that God exists in a physical sense? Some physical evidence would be nice. It might be fine for theist to believe that a being which created all that is, everything, has chosen to play hide and seek with us, to hide completely and thoroughly, but that seems an awfully thin framework on which to build an entire ontology, not to mention all the varied dogmas and doctrines.
Failing physical evidence, a simple logical argument proving the existence of such a creature. The theists have tried this for a few milenia and no one has come up with a convincing argument so far. I doubt anyone has a new twist on the very old debates. And please do not repeat the old "everything can't come from chance nonsense." The physical world with or without a Supreme being relies very little on chance events.
I'll tell you what won't convince me - arguments based on faith or feeling, the sort that basically boil down to, "well, I believe and so should you." You might see God in everything around you but I am not going to swallow your religion based on that any more that I would if an odd looking kid walked up saying "I see dead people." I did like the move, nevertheless.