Are the Old Testament and the New Testament contradictory?

 An issue that many Christians (and explorers of Christian faith) have struggled with is the apparent disparity between God as described in the Old Testament - who is (at times) a God of war and wrath - and the Lord as he reveals himself in the New Testament.  

I like Holdsworth's approach to this question, though it is by no means a "final" or complete "answer" to all of the difficulties.  I have often pointed out to people that, because the Bible is a progressive or unfolding revelation, you know more about the character and purposes of God by the end of the story than you knew at the beginning.  In other words, St. Peter knows more of the character of God than Abraham did - though Abraham really did know the real God.  

If you think of it like this, we don't get to the later, more complete revelation, without taking those earlier steps in the process: You don't get Jesus, the New Covenant, and the Moral Teachings of the Sermon on the Mount without first getting Moses and the Law.  And so it then is a bit self-contradictory to use Jesus and his teachings as a reason to reject the Old Covenant as being truly from God.  

That still doesn't solve the real difficulties of God who is depicted as commanding what we would call "crimes against humanity" when he tells the Israelites to wipe out the Canaanites completely, (and there are more complications in all of that than is often appreciated - including questions around our attempt to judge ancient & pre-Christian methods of warfare by modern standards, which are very influenced by Christianity), but it does make "space" to look again at the Old Testament as genuine revelation of God. 

Another aspect of how the Tradition has dealt with this question that Holdsworth doesn't get into here is the fact that many of those troublesome passages that horrify us today, were read allegorically by the Early Church Fathers.  They would say to think of the Canaanites as symbols of the sin and idolatry that reside in our hearts and must be completely rooted out.  This tradition of reading much of the Old Testament and the Psalms opens a new possibility of hearing a "word" in these passages despite our concerns.