It is written in Ephesians 4:3 "Be diligent to preserve the unity of the Spirit in the bond of peace".
Unity is a great theme in many of Paul's letters. And this is the burden the Lord has for His church too. When a human body dies, it begins to disintegrate. Our body is made of dust, and the bits of dust are held together because there is life in this body. The moment the life is gone, disintegration starts; and after a while, we find that the whole body has become dust. It is the same in a fellowship of believers.
When the believers in a church are disunited, we can be sure that death has already come in.
When a husband and wife are disunited, you know that death has already entered, even if they never divorce each other. Disintegration can start in a marriage within a day after they are married - with misunderstandings, tensions, quarrels, etc.
That can happen in a church too. A church usually starts with a few zealous brothers who come together with great zeal to build a pure work for the Lord. Very soon disunity comes in and death enters. We have to fight a battle constantly to preserve the unity of the Spirit - both in a marriage and in a church.
The wonderful thing about the human body is that all the little bits of dust that it is made of, are all united together so closely that one cannot see where they are joined. If the body is injured, certain processes start within it immediately to close up the skin. The body does not like any part of its skin to remain gaping and open. It starts working immediately to unite the separated parts of the skin. It is the same when a bone breaks. The body immediately starts working to unite it. No man on earth can join two bones together. A doctor can only place the broken parts of the bone next to each other. It is the body itself that joins those two parts together. The human body always works towards unity. That's how the Body of Christ also should function. When a church does not function like that, it is not representing the Body of Christ.
God is not building a bunch of holy individuals. He is building a Body. This is what Paul is speaking about in Ephesians 4. He urges us to "preserve this unity of the Spirit because there is one body."
When can we say that there is unity in a local body? "By the bond of peace" (Eph. 4:3). "The mind of the Spirit is peace" (Romans 8:6). When you think of a brother or sister, and your thoughts are thoughts of peace and rest towards him/her, then you know that there is unity between you and that person. But if you are even slightly agitated when you think of that person, then you can be sure that you are not united with that person. You may greet him with an exuberant, "Praise the Lord", but it is hypocritical.
Peace is the test. So the unity of the Spirit must be preserved in the bond of peace.