In my life, I've never said "God does not exist," but, rather, "I have yet to encounter a valid reason for believing the existence of God".
Why?
The Christian religion is premised upon two things: (1) the testimony of the Bible; (2) the historicity of Jesus.
1. The Bible contains countless errors and many documents that are known to have been forged (e.g., II Peter). It is not to be trusted. If the Bible is actually the word of a deity, then we can assume it would be infallible. It would have "the universality of the law of gravitation and the perfection of the arithmetical table" (Joseph Lewis). The Bible is far from perfect and thus lends itself to a multitude of interpretations. A perfect work would never lead anyone to this conclusion; an arithmetical table, for example, has only one interpretation.
I challenge a Christian to reconcile the two genealogies of Jesus with each other. I challenge a Christian to reconcile the genealogy of Matthew with itself (count the generations: there are not fourteen plus fourteen plus fourteen generations listed in Matthew -- although Matthew says that there are). I challenge a Christian to reconcile either genealogy with the Old Testament.
2. The earliest descriptions we have of Jesus Christ, those of St. Paul, give no details about where he lived, when he lived, or what he did. The only say that he lived sometime in the past, somewhere in the Palestine area, and that he died. This early testimony does not differ at all from the various myths that were floating around in those days -- myths that all Christians discount as fable.
It was only in about 90 C.E. that alleged details about Jesus' life begin to materialize and be circulated, in the form of Gospel stories. These details contradict one another, from Gospel to Gospel, because each Gospel was developed in a separate region and a separate sect. Mark was used to create Matthew and Luke, and each of the latter is clearly an improvement -- a correction, if you will -- of Mark. The Jesus character gets grander and more all-powerful with each succeeding Gospel. (For example, in Mark, Jesus could not heal certain people because of their unbelief, and had to use salves and other physical means. These are "corrected" in the later gospels, as it was unthinkable, at this later time, that Jesus would have such limitations.) John is so far out in left field that it barely squeaked by the ratification process several centuries later. (Yes. The canon of the Bible is the result of a vote: the books that were included in the Bible won, and the books that are not in the Bible lost.)
Only after there had been plenty of time for the Gospel fables to mature do we find references to specific details of Jesus' life (such as that he allegedly lived during the time of Pontius Pilate).
Thus, for me to even believe that a man named Jesus even lived during these times is a stretch. For me to accept the Gospel accounts at face value would compromise my sense of truthfulness. In other words, for me to say that these accounts are trustworthy in any sense would be for me to tell a lie. I have similar doubts about whether Mohammed and Moses ever existed. In other words, I can explain the existence of Christianity without there needing to have been a real Jesus, just as I can explain the existence of Judaism without there having been a Moses, just as I can explain the existence of Islam without needing to resort to a historical figure named Mohammed.
