The Old Testament contains many passages that are difficult to believe. There are of course problems with scientific, geographical, or historical details, the most famous of these being the six-day creation. These issues do not present severe difficulty except to the fundamentalist who insists on taking every word literally. Allegorical interpretations of the Genesis story have been part of mainstream Christian thought since long before the scientific age.
The more serious concerns with the Old Testament come from passages where either the “moral of the story” itself, or the life lesson that a believer could be expected to draw from a passage, or the depiction of the character of God, strikes us as terribly wrong. For example, in Numbers 5:11-29, we read that the Lord told Moses that wives accused of unfaithfulness should be subjected to a kind of trial by ordeal involving bitter water. In Deuteronomy 21:18-21, we read about how Moses taught (on God’s behalf, taken in context) that a stubborn and rebellious son should be stoned to death. While one could perhaps defend these things according to some theory of what kind of legal system best suited ancient people (not something I’m inclined to do), there are even more outrageous passages to be considered. In 1 Samuel 15:1-3, we read about God ordering genocide against the people of Amalek (even babies). Much of the Book of Joshua is about Israelites making war on their neighbors, on God’s orders, and showing no mercy.
All of this is enough to make atheists like Richard Dawkins call the God of the Old Testament “the most unpleasant character in all [of] fiction”. Christians stumbling across verses like this for the first time are often bewildered, and I suspect many have lost their faith because of not being able to make any sense of this problem.
If someone is already committed to the proposition that the entire Bible is absolutely without error, there are ways to defend God’s actions as seen in these passages. A couple of examples can be found here and here. I am not endorsing either of these explanations, and I find neither of them to be persuasive, but it should be kept in mind that these issues have been discussed for centuries and that it is possible for well-intentioned and thoughtful people to disagree on them.
Personally, I find the differences between God’s character as revealed in Jesus and God’s character as depicted in some of these passages to be too difficult to fully reconcile. My own view is that the Bible is not inerrant, and that the historical reliability of Biblical events varies. The New Testament stories are generally much more credible because they deal with a known period in history (the 1st century Roman world, especially the events in Palestine during the time of Pontius Pilate) and were written down during the lifetimes of those who witnessed the events. The Old Testament’s origins are much murkier and many of its narratives reference events that are difficult to place in a historical context. The Christian faith is centered on Jesus Christ; He is the likeness of God, and the truth of any particular Old Testament teaching should be tested in light of this.
One question that comes to mind is whether Christ taught that the Old Testament was free from error. Many Christians think so, but I’ve discussed this at length in this post and have concluded that no, He did not. Much of what Jesus taught turned some aspects of Old Testament teaching on its head.
Another question that comes to mind is that if the Old Testament contains errors, why should one not think there are errors in the New Testament as well? My answer to this is one does not have to assume the New Testament to be perfect in order to conclude that it is true in its essentials. While some may use the Old Testament to prove the New, and use the Bible as a whole to prove the existence of God, my own reasons for believing work in reverse order. First and foremost, I believe in God, Creator of the universe, the source of consciousness and the moral law, and I have explained why here. I believe in Jesus both because He “rings true” with my heart’s understanding of what God is like, and because eventual triumph of His message over the Roman Empire is best explained by taking the New Testament as non-fiction. I’ve explored this in more depth in this post and this post about comparing Christianity to other religions, and have pulled everything together in a page called “Why Believe?“.