From my essay Why I Am No Longer a Catholic, which discusses the reasons for my apostasy in detail:
The story of how I entered the Church is a very common one: my Catholic parents had me baptized not long after my birth, brought me to church each Sunday, and sent me to formal religious education at the church until I made confirmation at the age of thirteen. As do many children, I simply accepted what I was told by adults in essentially all matters without questioning it, and this certainly extended to what I was told about God and religion.I considered myself agnostic after my apostasy on 3 October 2004, and I became an atheist a few months later. That's where I am today, happy and free.
While my enthusiasm for religion waxed and waned, I cannot recall any doubts of any significance until the summer after I graduated from high school, at the age of seventeen. For the next eight years I struggled from time to time with these doubts, and reassured myself with various reasons for believing the Catholic faith true.