The Old Testament is a foundation of context used to illustrate God's frustration with people who oppose him. The New Testament describes the life, Death, And effects of Jesus. He was a noble, Honorable people (compared to the false prophets and deceivers of the Old Testament) who introduced the idea of feminism into the world, Helped the children wherever he saw them, And after hours of torture and possibly upwards of a day fixed to the cross he was nailed to, He said in his dying breath, "Father, Forgive them, For they know not what they do. " What rules from the Old Testament Jesus did not directly oppose, His personality and message did. So, Christians follow the New Testament more closely than the Old Testament because the New testament is what separates them from the Jewish and Islamic faiths.
Christians generally follow Christianity, And many believe the central idea that Jesus is the son of God, And together with him and the Holy Spirit, They make up the Holy Trinity. If you believe that, Then yes, You are following Christianity. As for the Old Testament laws of slavery, Rape, And murder, Those laws have been done away with by the New Testament. Those laws are for the Jews.
It is a common fact that, Out of the 98 books written as the original Bible, Only 66 books is seen in the Bible today and even out of the 66 books, A lot of things has been taking out and others has equally been tempered with, Although it is written that, Nothing should be taken out and nothing should be added to it. However, There are still traces that shows the real people in that Book and it is therefore only these people who know and equally comprehend the Word in it that knows the God in question and their existence and not the so called Christians of today. Jah Liveth!
In the United States of America, If you practice true Old & New testament practices, You will be arrested and put in prison for a long time. Polygamy, Incest, Sex slavery, Rape, And murder are all illegal in most countries thank God. If Christian's actually read there Bible's and researched some of the controversy of the Bible, They wouldn't be Christian's. Unless of course they were psychopaths and liked to worship a God that approves of those behaviors. So thankfully no, Christian's don't do as there told