God's people are meant to be one people, undivided, joined together by Christ into one body —one in doctrine, direction, and devotion. Jesus prayed that his disciples would be one, just as he and his Father are one (John 17:21).
First we should consider this unity as a unity arising from truth. There is no value in achieving unity, if what we agree upon is a falsehood. There is no good in a bond of love, if we love a lie. We need to love and teach the truth.
The Father, the Son, and the Holy Spirit all speak one and the same truth without any disagreement
The apostle Paul made their truth his truth, and that's the truth he taught in all his ministry. Furthermore he insisted that all the apostles, and everyone else, speak that same truth and have no divisions among them (1Corinthians 1:10).
Paul says of some who were astray, "...they refused the love of the truth that would have saved them. For this reason God will send them a powerful delusion so that they believe the lie, " (2Thessalonians 2:10-11 BSB).
They were united by their belief in that lie, but that unity was worthless because it arose from falsehood not from truth.
For centuries there have been doctrinal divisions separating those who profess to be Christians. More divisions are being added as time goes by.
"For the time will come when they will not endure sound doctrine, but according to their own desires, because they have itching ears, they will heap up for themselves teachers;" (2Timothy 4:3 NJKV)
One name for Christianity is "The Way" (Acts 24:22). This suggests that true Christianity has a certain direction in which its followers must walk. There is one right road that goes in that right direction. True Christians should not be going along other roads that take them in different directions.
Consider the saints of old, such as Enoch, Noah, Abraham, Moses, and David. God gave them each a pathway to follow. Those old paths led them all in the same direction. In their paths lay the good way, the way that leads to life everlasting. God invited everyone to walk in that good way, but many refused.
"Thus says the LORD: "Stand in the ways and see, And ask for the old paths, where the good way is, And walk in it; Then you will find rest for your souls. But they said, 'We will not walk in it.'" (Jeremiah 6:16 NKJV).
When Christ came, he said, "I am the way..." (John 14:4-6). He is the good way, and following him we walk in God’s light heavenward (1John 1:8).
However, as Paul says, "In later times, some will depart from the faith by devoting themselves to deceitful spirits and teachings of demons" (1Timothy 4:1 ESV).
Today many who are called Christians follow a prophet, an idealogy, a sectarian creed, a contemporary fashion, a miraculous sign, or a false doctrine. So they are turned in the wrong direction, away from following Jesus in "the unity of the Spirit" (Ephesians 4:3).
The first Christians "were all together, fiercely united... and they continued steadfastly..." (Acts 2:1,42). In other words they devoted themselves to the way that Jesus had shown them.
If Christians lose their devotion to the Way, and their love for truth grows cold, their unity suffers because they have stopped "endeavouring to keep the unity of the Spirit..." (Ephesians 4:3).
Jesus spoke of that loss of endeavour and devotion. He said, "Because iniquity shall abound, the love of many shall wax cold" (Matthew 24:12).
Christians today need to restore the original doctrine, direction, and devotion of the first Christians. That will break down division, and bring back the unity of God’s people.