Firstly, by the gradual realisation that the sect I belonged to was a racket. Secondly, by the observation that all other Christian sects were to a greater or lesser extent rackets, too. Thirdly, by the simple conclusion that a book (the Bible) which can produce belief systems as far apart as Roman Catholicism and the Plymouth Brethren must be very ambiguous and that if its authorship were divine then the author would have far more in common with Loki and Hermes than anything resembling the Christian description of god as all-knowing, all-loving etc.
Then I looked at its place of origin and its historical context. Christianity and its Semitic bedfellows are obsessed with obeisance, worship, prostration, obedience and the punishment of those who offend. With unintentional irony Christians call God “Our Father”. There is nothing fatherly to be seen in a deity who wants his children to grovel in front of him. The Christian God, as that of the Jews and the Muslims, is made (oh so obviously) in the image of Middle Eastern autocrats of that time and later and shares the same moral defects of those reduced beings: vengefulness, pride, caprice and cruelty.
Then I considered the central tenet of Christianity, redemption by blood sacrifice. Who apart from cruel and ignorant people who slaughter animals to propitiate their gods could not find this doctrine disgusting? Is any further argument needed?
And lastly there’s the issue of theodicy – why God allows suffering. Well, I won’t argue with the great David Hume who allowed that pain and suffering COULD be compatible with the all-loving, omnipotent god of the Christians. But the went on to say that simply by observing the world you would never infer such a being existed, would you?