... Do Christians believe in the OT anymore? ...
Every person is unique. There is no thing that all people that call themselves Christians believe the same. In any large enough group of people, you will find every kind of person. Since
you are asking, I assume you have not resolved the question in your mind.
Don't believe "in" the OT or NT, believe in God. Seek God not the Bible. Seek to know about God through the Bible. Seek to know God himself similarly to how you get to know a visible person better. Fulfill your side of a good relationship between a person and God and he will fulfill his side (with all the capabilities that God has).
The nature of the creator of the universe has never changed nor will it ever. However, he doesn't reveal his nature in one big massive revelation to us. (Fortunately:
But, ” he said, “you cannot see my face, for no one may see me and live.” Exodus 33:20, 1984 NIV) He reveals himself a bit at a time to an individual. The Bible is our #1 source of what he is like and who he is. We might read the whole Bible, but we won't understand the whole Bible afterward. We must persist in seeking God if we want to know about him better and to know him better.
Seek the LORD while he may be found; call on him while he is near. (Isaiah 55:6, 1984 NIV)
The God who revealed himself through the writers of the OT is the same God who revealed himself through the NT. His behavior in both parts of the Bible is consistent. It is the circumstances that vary. The story of the Jews in the OT is not all smiles, because when God started with them, they had hard hearts (unwilling to acknowledge God as GOD, unwilling to change their ways, unwilling to believe and trust what God said). It is only through God's grace that they started drawing closer to God. When they ought to have understood their calling as God's people, they turned away from it. Again and again.
God has always wanted what is best for all of mankind. But not intervening as he did meant everyone would become like the people prior to Noah as a result of their own choices.
This is the verdict: Light has come into the world, but men loved darkness instead of light because their deeds were evil. (bold mine, John 3:19, 1984 NIV)
The LORD saw how great man’s wickedness on the earth had become, and that every inclination of the thoughts of his heart was only evil all the time. (bold mine, Genesis 6:5, 1984 NIV)
It is essential to understand Romans 5:12-14:
Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all men, because all sinned—for before the law was given, sin was in the world. But sin is not taken into account when there is no law. Nevertheless, death reigned from the time of Adam to the time of Moses, even over those who did not sin by breaking a command, as did Adam, who was a pattern of the one to come. (bold mine, Romans 5:12-14, 1984 NIV)
It is the nature of the creation that sins bring death whether God passes judgment on someone or not. All he told mankind was not to eat of the fruit of a tree. Easy to understand. Easier to do than we are familiar with, since this was before sin entered the world. Only believing words contradictory to what God had already said created a stumbling block. He gave more commands after Adam sinned to save people and keep them from propagating sin, but people continued to reject God. As the sin of mankind increased, and since God loved us, he chose to intervene. God chose a group of people (the Jews) through whom the world would learn to turn away from the actions that would demand just punishment from God. (The Jews agreed because it would save them from slavery.) He gave the Jews the Law through Moses to reveal to them which things they were doing were resulting in eternal death.
... through the law we become conscious of sin. (Romans 3:20b, 1984 NIV)
They were supposed to obey and spread the saving truth. But they couldn't even get the first part right. Every so-called "harsh" act of God in the Old Testament was to stem the tide of sin that was continually spreading to give the descendants of those alive a better chance to avoid morally necessary punishment. The harshness is not because God is harsh, it is because the results of sin are far worse than we, today, usually imagine. Mankind basically said they didn't want God even as their judge. So he let some of sin's effects past his normal protection so we could get a taste of what the consequences of what we were doing were. The effects of sin are what is harsh, and to be honest, the worst horror you see in the OT is peanuts compared to sin's full and true effect. Whenever God intervened, if the people were of noble spirit, it turned them away from sin for a time. However, eventually they would turn back to sin, and God out of his love, would intervene again.
Some would like to say that God created us too predisposed to sin, but God didn't choose how predisposed to sin we would be. He made us perfect (in His image). There was no way to make humans better than he did. If humans had never sinned, they would have grown to be like God more and more. Some might say God created the universe too much against us, but he made the universe perfectly, too. It perfectly reflected the nature of its painter. There was no way to make the nature of the universe better for us.
God is love (from 1 John 4:8 and 1 John 4:16). Because of his own nature, God still wanted to save us in spite of the fact that every person that ever lived sinned. God's love is within himself. It does not depend on things (like people) external to him. Within God's nature was an allowance that someone of the same family could pay for the debt of another family member. (See
another of my posts on this web site for more detail.) God became man so that the fact that he was the Father of all mankind would become Truth in the world. Because he had the ability to pay for our sins (which we did not), He paid for them. It cost him everything including his life in a sense I don't understand, but I can say one thing: Before Jesus came, God was all Spirit. After He became man, his honor was lowered in some way for our sake, because he became a part of something he created (a created thing deserves less honor than its creator, Hebrews 3:3).
When Jesus was born, the author of mankind became the highest person in all mankind (the Father of us all). All the authority that mankind had went to Jesus, the head of mankind. At that point he had the authority to save all of those who would recognize his authority over them (Hebrews 7:25). The fact that all people were his children does not have the necessary moral component for salvation unless a person accepts their place in God's family. Unless we accept Jesus as our Lord, we have not given him the right to save us. As a result, he is
the Savior of all men [he died for all]
, and especially of those who believe. (comment in brackets mine, 1 Timothy 4:10, 1984 NIV)
Praise God!
“You are worthy, our Lord and God, to receive glory and honor and power, for you created all things, and by your will they were created and have their being.” (Revelation 4:11, 1984 NIV)