John 17:21

“That they may all be one, even as you Father are in me, and I in you, that they also may be in us, that the world may believe that you sent me.”

His prayer is that the unity which He has requested for the Apostles may also be experienced by His people as a whole. That unity He likens to the unity between Himself and His Father, a unity of purpose and action, of love and truth, which will be theirs as they abide in Him and the Father. This portrayal of unity and love will then make its impression on the world so that the world will believe that Jesus Christ came from the Father. Here of course He means by ‘the world' that part of ‘the world' (that is, of the non-believing world) that sees Christians active in a unity of love.

For some considerable time that unity did impress the world. They said, ‘see how these Christians love one another'. And this was enhanced by the persecution that drove Christians together. Even today whenever the hearts of Christians are firmly set on Christ rather than on the church there is a unity and love which is remarkable to behold. The more we ‘abide in Christ', the more that oneness is seen. But let us get on to cold doctrine alone and that unity becomes conflict. There is no greater divider than enthusiasm for some secondary interpretation or doctrine, or our own individual interpretation of Scripture.

It is clear that Jesus was as much concerned for the open revealing of this spiritual oneness as for anything else, for He continually underlines it. Christians will inevitably disagree on doctrine, on views of the scriptures, on church government and on many daily practises, but when they have allowed this to destroy essential oneness with all Christians who truly believe in the LORD Jesus Christ, they have committed a great sin. They have denied their birthright and brought shame on Christ. If men are one with the Father and the Son, then they are one with each other, and must love one another, and  must show it, for how else is the world to believe?. This is the ‘unity of the Spirit' (Ephesians 4:3). It is the result of the Father's love in them (v. 26) It does not mean compromising what they see as the truth, it means that they love one another while disagreeing, because they are one in Him. That is what matters above all.

The basis of this unity is that they have heard and received the word of the Apostles. It is a unity based on apostolic teaching, an assumption illustrated in 1 John 2:19 where it results in their “abiding in the Father and the Son” (abiding in the truth of the Triune God), as long as what they have “heard from the beginning” (the truth presented by Apostolic men) abides in them (1 John 2:24). There are a few essential truths which determine a man's position before God. If a man believes in Jesus as uniquely God's Son, and in the fact that His work on the cross, and that alone, somehow brings him an undeserved forgiveness, and responds to God on the basis of this, is he not made one with the Father? Then he must be embraced in the circle of Christian love however differently he may view more detailed interpretations.

God's final purpose is to reconcile all things to Himself (Colossians 1:20) and to bring all things into harmony in and through Christ (Ephesians 1:10), removing the rebellion and disharmony that man has introduced into creation (Romans 8:1). The church was intended to be the firstfruits, the outward sign that God's purposes were on the way to fulfilment. We shame Him when we fight with each other.