For the purposes of this article, let us first assume for argument's sake that Christianity is the true path to our eternal salvation. Let us discuss atheism at another time.
Christianity is largely based on what we read in the Bible, and most major denominations of "mainstream Christianity" claim to take their beliefs directly from the Bible. Moreover, these denominations tend to either take the whole Bible as the literal truth, or pick and choose which events recorded in the Bible are literal, or are metaphorical — and I believe it is safe to say that no two denominations, perhaps even no two people, interpret the Bible in the same way. To make matters worse, there is no translation of the Bible extant that is free from translational errors from the Greek, which itself was translated from the Hebrew and the Aramaic.
For example, the earliest major translation to Latin was the Latin Vulgate, by the Catholic Saint Jerome. Before this translation, there was no Biblical difference between "angels" and "messengers" in the Hebrew, the Aramaic, or the Greek. I can't remember the Aramaic, but in Hebrew, they were both "mal'ak", and in Greek they were both "aggelos." The spies of Rahab were referred to as angels in both the Old and the New Testaments. In other words, in Biblical times "angel" meant "messenger" or "someone sent to do something" and not much more. The Catholic Saint Jerome decided to differentiate between those sent by God and those who were not sent by God, by referring to the former as "angels" and the latter as "messengers" ... but in Latin, of course.
Then there are the stories of Genesis, which most who claim Christianity will say means that God created everything about 6,000 years ago; of Noah's flood, which anyone with a passing knowledge of geology knows to be a sheer impossibility; and of the 42 children who were killed by a she-bear for mocking a prophet. There are many other stories in the Bible that also make no sense to the cynical mind.