John 17:21

That they all may be one] A leading passage on the unity of the Church. The centre of unity is not on earth but in heaven. Christians are 'one,' because they are spiritually united to the Father and the Son, whose divine life and blessed union they share through the faith that gives eternal life (John 3:16, etc.), and through believing participation in the sacraments (John 3:5; John 6:56; 1 Corinthians 10:16; 1 Corinthians 12:13). In this, its deepest sense, the unity of the Church cannot be broken by outward divisions. But inward unity ought also to show itself in visible outward unity, 'that the world may know that Thou hast sent Me.' Hence every Christian is bound to pray and work for the reunion of Christendom.