When I am certain of my opinions and beliefs, I am intensely certain. If I were certain about this, I would be one of the "militant atheists" I've heard so much about, and I wouldn't be involved in this hugely rewarding internal struggle (and trust me, you've only gotten the tip of the iceberg so far).
I do mean that; being Catholic was never as rewarding as my rejection of it has been. Questioning God has been one of the most rewarding experiences of my [albeit short] life.
Incidentally, I said that last bit to my mom the other day and she responded with "Well, maybe we should sit down and question God together." Will I sit down with my uber-Catholic mother and question her reason for existence? I will--because my mother always listens to me, always supports me, and never tells me I'm going to hell. She never forced religion on me. Ever.
Which is another reason it's odd that I left the church. A lot of people end up atheists as an inverse response to intense familial and societal pressure, but I just kind of got there on my own.
When I stop to think about it, the odds against me ending up where I am today are enormous. I think you'll see that point illustrated time and time again.