John 17:21 'That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me.'
Jesus is praying for all believers to be one as He and the Father are one.
This goes far beyond what many promote as unity today. Paul besought the Corinthians to 'all speak the same thing, and that there be no divisions among you; but that ye be perfectly joined together in the same mind and in the same judgment' (1 Cor. 1:10). This is God's standard of unity.
This oneness among believers is what Jesus said would cause the world to know that Jesus is the Son of God. This is the greatest tool for evangelism that the church has. The only way that Christ's body will be one as the Father and Jesus are One is through God's kind of love. No wonder Satan tries to get believers to go at each others' throats. We spend billions of dollars a year on evangelism through television, radio, conventions and crusades, yet, the world is not evangelized because the body of Christ is not united in love.
There is very little unity among believers today. This has occured over thousands of years and it probably won't be fixed over night. We must strive towards unity but not be overwhelmed by the problem. All Christians have already been joined to each other through the body of Christ, and God the Father sees us all as His children. All divisions among Christians are made by man, not God. For a brief period of time, the church enjoyed unity on earth. Regardless of what strife and division has occurred, all believers are still one in Christ and will live in perfect oneness throughout eternity. We are now one in Spirit. We just need to experience that unity here on earth.
'Thy kingdom come. Thy will be done in earth as it is in heaven' (Mt. 6:10).