It's normal to question God's existence when you are at the depths of sorrow and see no hope in sight.* It's also normal to question His existence when you are young and thinking you're the master of your fate.
I was 20 when I decided there was no God and I no longer needed to bother going to church. (It was so boring! And all they really wanted was money, right?) There wasn't any big catalyst or anything; I just decided that what I'd been taught in my Catholic upbringing was probably just a manipulation tactic and I would be just fine without "faith." I didn't know anything about this "Jesus guy" and figured I could handle my life on my own. And for the next year or two, I did.
I'm sorry to be vague, but something happened to me around that time, something that literally brought me to my knees (in spite of myself) and left me crying out for help. (Well, sobbing, really, with lots of choking, and snot. Good thing I was in the shower)
He answered, loud and clear, in the form of an unexpected event a day or two later and by placing two people in my path whose revelations to me made me realize that I was not alone in my suffering. I've been a believer ever since and have a peace in me that I consider the greatest gift I have ever received.
I still struggle with many unknowns in this world, like why absolutely horrific things happen to very innocent children** and the explanation for natural disasters that kill thousands, etc. I still don't really "know" Jesus in the way some might insist that I need to, but my faith is strong. I enjoy church now (though I don't go as often as I should); I find the rituals of my faith and the sense of community comforting. I've been at church in the depths of sorrow (when we've lost family members) and in the same church at times of celebration, with my heart full of joy. It's home to me.
I respect you for your own beliefs (even if you don't believe in God), and I wonder (regardless of your beliefs, and if you feel like sharing) when/how did you know for sure?
*P.S. While Molly's (and her ex-husband Mike's) experience does have me feeling introspective and a little melancholy lately, this statement is in no way representative of how she is feeling right now. I am not talking about Molly here. [Also, the trial starts tomorrow and, with any luck, will end Friday.]
**Again, not referring to the Molly situation.