No Christian believes in the entire bible, OT or NT. If they did, they would almost certainly be in jail for stoning someone to death, keeping slaves, or something.
Christians believe in all those parts of the bible that they agree with, but interpret (ignore) the parts they don't agree with.
For instance, how many Christians work on the Sabbath?
There are many christians who fail to "rightly divide or interpret correctly". This is not news since the bible was written in other languages, and anyone who speaks more than one language understands that true meanings can get lost in interpretations. However I can say this:
One scripture washes the other, and before you can assume to know something you must consider all of it's influences. For example the commandment was "remember the sabbath to keep it holy", yet MEN added to this law by saying you can't work. This is the same case we find with Catholics claiming that priests can't be married, when the bible clearly demonstrates that even the Apostles of Christ had wives and were first fishermen by trade.
Can you or anyone else find the scripture saying that holy men of god can't have wives? Thus it's a necessity to consider how people distort and interpret before claiming something is what the bible says or what "Christians believe in general".
As for stoning, that would be considered what we call the "death penalty" today. In today's system we have judges, and they decide which crimes are punishable by death. Some people are against it and some are for it, however my stance is let God decide who lives or dies because of the scripture that says we can not add one hair to a mans head so we have no right to take his life. In simple terms God is saying we don't have the right to destroy something that we do not own and is valuable to him. People are also living souls as spoken in Genesis so we are different from animals or plants.
I make that conclusion because when animals were created they are not called living souls in the bible. They are also called "the creation" in the book of Romans. Only mankind is given the ability to be the "sons of god" as spoken in Romans. No animals are proclaimed to have the ability to accept Christ. So in light of God accepting us through Christ as sons, allows me to deduct that killing a potential son of God can be related to breaking your moms favorite vase. That's a no no even if the vase has been particularly naughty.. lol.
There are many other scriptures, such as when Christ was asked "what is the greatest commandment" he stated 2. Love your neighbor as yourself, and love God with all your heart. If you truly love people you will not desire to see them die, even if they did something horrible. Mothers are a great example of this since they would rather see their child in prison rather than killed. This was the basis for the wisdom of King Solomon when he determined that the true mother of the baby was the one who would rather give her child to the woman who had stolen it rather than see it cut in half.
So you see the bible is not what it seems when looking at only one scripture, you must rightly divide the word and dig deeper to find it's true meanings. You will also find that God simply " allowed certain things to go on " for a time because of the ignorance of man.
I concluded this by what Paul says in the new testament about the church at Corinth: " I can not give you meat, but only milk of the word ", because he knew that the minds and hearts of the people there were not yet able to digest the more complex reasoning's of God.
This is how anyone can maintain faith, you must consider that if things don't add up then you're not getting the full understanding. So then you must endeavor to learn more to get the truth. " in all thy getting, get an understanding " Proverbs 4:7
About Slavery:
I'll be very simple, after all of the law and prophets of the old testament, God himself spoke from heaven with the loud voice on the day Jesus (Yeshua in hebrew) was baptized and said: This is my son in whom I'm well pleased "HEAR HIM !"
And what did Jesus say? Love your neighbor as yourself. Now as we have seen throughout history in secular history books, slavery is not the same consistently. There was a difference between a slave who was forced into slavery through "kidnapping" and a slave who owed a dept and became a servant to repay it. The scripture also states that the debtor is "slave" to the lender. Proverbs 22:7
However regardless of how you interpret the word slave you still have to answer to Christ as your final judge according to his place in glory and heaven. The scripture says that his name is above every other name in heaven or earth, and every knee shall bow and tongue must confess he is Lord. So slavery before the Civil War in america would not fall in line with "love your neighbor, and since those slaves were kidnapped or taken by force, they could not be in debt to their masters then could they?
Scripture: "He that loveth not knoweth not, God is love" 1 John 4:8
Romans 8:14 "For as many as are led by the spirit of God, they are the sons of God."
If you are led by the spirit of God who is love, how could you take a man by force from his family and say that he is your slave. This is kidnapping and a violent act under almost any bill of rights or constitution of any nation, yet people are still questioning (out of ignorance) if God is for or against it.
Kidnapping also falls under the commandment "though shalt not steal" Gotcha !!!
So people who were made slaves by forces and not by rights through debt even in the old testament would still be unlawful under Gods 10 commandments. Every person belongs to some family even if their family is dead or alive. And if you can just steal people then why not add houses, money, cars, and wives to that loaded word "slave"?