Whether it's eternal security or some other basic truth of scripture how come no two Christians will agree with the Bible?
False premise. Matthew 12:30.
Why all the divisions in the church and among Christians?
1 Corinthians 14:33, James 4:1.
What is the truth about scripture?
Eg: 1 John 4:1, 2 Peter 2:1-2, 2 Thessalonians 2:3.
There has been an apostasy - a great falling away from the original faith. Jesus said this in Luke 19:13-15 - the parable, when He went to receive His crown in heaven and then to return, He left The Kingdom in the hands of servants - but His subjects hated Him, sent a delegation after Him, to say "we will not have this man to reign over us".
St. Paul warns too: "now even more in my absence—work out your salvation with fear and trembling" - "I have no one else like Timothy who will genuinely care about you—for all seek after their own interests, not those of Christ Jesus".
Most Christians (not all of them, by miracle) have been deceived by the false doctrines of apostate teachers before they have come to maturity, and therefore they are under the spell of the devil - the father of lies. They twist the scriptures to make them fit with their predefined doctrinal views, that they did not from from the bible, but from subjecting themselves to the distorted teachings of the apostates at the pulpits.
The truth is the thing that Jesus said would set us free, and there is unity in the truth when we are united in our love of it. But He also said that we would need to keep His commandments in order to know the truth that would set us free.. and that is what they don't do (eg: want to say that a baby will go to hell because of what Adam did? then you do not teach justice in God's name).
There are some people, calling themselves Christian, who don't actually love the truth at all - they just like to argue (possibly something within you will feel like arguing with me now, and that is how it happens - that is the flesh, ego, pride).
The false Christians have followed some idea that made them think that they have to make a covenant with death in order to escape eternal torment (Isaiah 28:15-19).