Kidder asked in a comment what I think brought about Old Testament Christianity. I think it was several factors, but I think the transformation of the vast majority of Christianity to this viewpoint is one of the major watersheds and I think needs to be covered in its own post.
1. Enter Politics - One of the first steps on the road to Old Testament Christianity(OTC) was that Christianity changed from being just a group of people gathered together in a faith to a state endorsed and supported religion. Once Constantine endorsed the Christian Church, people flocked to it in droves for power and position. Money also flowed into the Church unlike had in the past. Small communities were transformed into large organizations, which breeds a whole new type of creature. Large groups have to be controlled. One tried and true human method is fear and FEAR for your immortal soul is a very strong fear.
2. Christ as Warrior/Ruler - Another aspect of this change came from the fact that Christ changed from being the King of Heaven to the King of this world. I am a Roman general, why would I follow a God who says we shouldn't fight or kill. Here the church changed to say it was okay to kill unbelievers and others if you were doing the churches will. This change brings forward the Martial Christ. Some of this comes from reading and interpreting the writings of Paul. Paul and other believers us military symbolism to explain concepts of what they mean in a Spiritual battle, unfortunately this can then be recatagorized in real terms. You have only to look at the pope to see this change. He is not just a spiritual leader, but a temporal one as well. Also look at England, where the Queen is Head of the Church of England (now more ceremonial, but not to Henry VIII it wasn't). Finally under this header, you cannot forget the Crusades. While at one hand a testament to power of faith that people could move whole armies to liberate conquer a land. You have once again Christ being re-cast in a martial mood. The Crusades represent the highwater mark of Church authority and zeal.
3. Conversion of Rural Populations - At the end of the 300s and 400s, you see the Christian faith spreading to more rural populations as well as the barbarians invasions. Christ is once again reclassified to make him understandable to these people. There is a lot of evidence that fundamental changes in the church and how it acted came about because of the influx of less urbanized members. An urban church based in a Greco-Roman setting would not appeal or attract followers outside that setting easily. Several books have been written in the last 20 years covering this period and the transformation of the church.
4. Councils and Edicts - Over the course of the last 2000 years, we have increasing lived in a church transformed by Edicts and Councils on how people should worship. Much of this having more to do with prevailing social viewpoints than anything else. Along the way earlier versions of the Christian experience are scattered around (Ethopian; Coptic; Celtic). These different versions show some of the changes that Christian belief has gone through as it passed through the centuries. This has even influenced translation of the scriptures.
NOTE: The Old Testament was written and re-written at least a dozen times before it entered the modern era. The New Testament was not first collected till almost around 200AD and people still debate what is canon there. For example, you can look at the "King James" Bible and see some of the prejudices of the translators there because many of them had axes to grind with the upstart Scottish King. These translations can change the emphasis of Christian belief.
While this may be a very high overview and a bit scattershot, I hope it makes some sense. I hope that as time marches forward, New Testament Christianity will come more to the forefront of the Faith. You can see it in some writtings by various theologians, but it still a very small segment and perhaps always will be. The vast majority of people want a God who sits up in Heaven and promises to bring them justice versus their oppressors. They want a God who makes the world a simple place, with only their viewpoint being dominant. Sadly this I think is true of all religions all over the world.