What is Christian unity?
Unity can be defined as the state of forming a complete and pleasing whole. It is essentially being in a state of agreement. Christian unity can then be defined as a state of agreement amongst the individual members of Christ's body (the Church) with the end goal of forming a complete and pleasing whole to the perfect groom, Jesus.
What are the causes of Christian disunity?
In a broad sense: Sin - in the lives of those who are sinned against and of those who sin. When sin isn't dealt with and resolved properly in the body, it often leads to schisms within the body. In finer detail it would be a lack of humility and a lack of love within the body itself.
How are we meant to resolve Christian disunity?
In 1 Corinthians 5 Paul asks the believers at the Church in Corinth to put out from among them one who had fallen into sin. In the 2nd epistle to the Corinthians he explains why this was necessary: He did it out of much love for them. He did it with much affliction and anguish of heart (2 Corinthians 2:4). In essence it pained him to issue such a form of punishment, but it was necessary. He goes on to explain how the brother overtaken by sin had repented and was indeed sorrowful and pointed out their need to forgive him and embrace him back as a brother. He urged them to comfort him lest he be swallowed up in sorrow. He also asks them to reaffirm their love for him. Perhaps you are aware of people who have fallen into sin and have been repentant but are too ashamed to come back to church.
Have you reaffirmed your love to them, or have they become consumed in their sorrow and embroiled in bitterness? Have you comforted them? In Hebrews 12:12-15 we are encouraged to strengthen believers who are discouraged and defeated so that they may not fall away but that they may be healed. This is in order to make the body whole and complete. We are also challenged to care for the church of God lest anyone be separated from grace (which is God's tool to heal and restore us and also that which empowers us to kill sin). Also, this is so that there is no bitterness which can cause Church division.
What if the issues just can not be resolved?
Pursue love. 1 Corinthians 13:7 tells us that love bears all things, believes all things, hopes all things, endures all things. Love bears all things. The word 'bear' means to cover over with silence the errors and faults of others. Is this my attitude towards other believers? Is it yours? Love endures all things. The word 'endure' means to bear ill treatment bravely and calmly. Pray for this. Finally, we are the body of Christ, all members of one another. If one man is down, we double back to go get him. (Galatians 6:1&2). Ephesians 4:26 reminds us to not let the sun go down on our anger. Be angry, and do not sin. In essence: In your anger, do not treat the brethren like garbage. Verse 29 encourages us not to let corrupt speech come from us - only that which is necessary for edification, that it may impart grace to the hearers. Ephesians 4:32: 'And be kind to one another, tenderhearted, forgiving one another, even as God in Christ forgave you.'