I think I shared mine once, but it was at least 10 pages back, so...
I was raised catholic, even attending catholic school (k-8), for a number of years. i used to be a rather avid believer, one who would often pray, attend church with a fervor, etc. But as I grew older, 12 if I had to put a year on it, I began to doubt it... and on this, I think all theists do at some point. just not all make such a turn about as I did. Anyway...
At first it was just a gentle questioning: "Why does the Pope, a man, get to change and adapt God's law?". I rationalized this as something along the lines of "Well, this is just a man made law, something not condoned by God.". Yet, as I'm sure you see, this begs another question: "If this is god's church, which he established and St. Peter is it's rock, why does this all powerful God condone this pollution of his message?". Which was still on the whole a questioning of man's interpretation, but still slowly approaching a questioning of the concept of God.
As I grew older I began to question evermore. This particularly accelerated when I moved to public school, as a catholic school is very much a sheltered environment. Such as "Why does this loving god let utterly horrible things happen to what he has created, and what he loves?", or, "Jesus once said: 'He that does not abide in me is like a branch cut from the vine, and withers. And the branches are gathered and cast into the fire.'. Now, if this is the son of god, and the son of a loving all forgiving god, wouldn't he be forgiving of his creations, whom are in rebellion to their creator according to the bible?". Perhaps this is man's pollution of god's word, or some other rationalization like "He works in mysterious ways." which always struck me as a cop out. Another way I've heard it rationalized is "It's like trying to explain to an ant how a tv works.". However this implies that the ant (man) is worshiping something it doesn't understand (god). Is it because the ant fears it, or that it can't explain it, so places some sort of contrived explanation around it. And we as bystanders can see this, and know full and well the television isn't a god, and can be explained very easily. I have yet to come across a good explanation for this.
Then, there are contradictions within the bible itself. Take Genesis, the story of Cain and Able to be specific. The short version is that god created Adam and Eve, Cain, Seth, and able, and seven women. And by this, these are the only people in the world. Then of course, Cain kills Able, and for this, he is banished. But hang on a moment.... He is banished, to live with the people in the Land of Nod.
Or let's look at Soddom and Gommora. When two angels came to talk to Lot, the one godly man, a mob appears and tries to rape the angels. To prevent this, Lot, the godly man, offers to the mob his own daughters to rape.
Then, there are some general ones, such as "Thus says the Lord, God of Israel: Put everyman a sword by his side, and slay everyman his brother.".
Now, let's get to the elephant in the room... The gospels, ie: the story of Jesus. There are inconsistencies within them. Such as, the virgin birth is only in two of the gospels. When he is crucified, in one before he dies he yells: "God! Why have you forsaken me!". In another, he yells: "Lord, into your hands I commend my spirit.". Not to mention the fact that these were written 150 or so years after Jesus died.
And then, there is the fact that this story, Jesus', was seen throughout the Mediterranean area for millenia prior to the gospels. Particularly in the Egyptian Book of the Dead. Which it may interest you all to know predates the Bible by 14 or so centuries. Here's the short version of the story:
Written in 1280 b.c., it describes the Egyptian god Horus. Horus was the son of the god Osiris, born unto men of a virgin mother. He was baptized in a river by Anup the Baptizer, Anup later being beheaded. Horus, before preaching and performing miracles, was tempted in the desert while alone, by the Egyptian god of the dead. He then gathered twelve disciples to him, and preached, and performed miracles across the land. He healed the sick, the blind, and cast out demons in the name of god (Osiris), and also walking on water was a noteworthy miracle. He also raised Asar from the dead. Asar, translated from Egyptian into Hebrew, means Lazarus. He was crucified on a hill, with two other men; 3 days after he died and was buried, two maidens announced that Horus, savior of humanity, had been resurrected.
This, in a nut shell, is why I unashamedly define myself as atheist. Doubt me as you will, hate me if you just. But if you've ever wanted to know why/how some people can be atheist... Here you are.