John 17:21

That they may all be one; even 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 didst send me.

That they may all be one ... is a prayer for Christian unity, the great reason for Christ's desire for such unity being immediately stated, "that the world may believe that thou didst send me."

In answering the question of how the believers' unity could effect the conversion of the world, Milligan said, "This would be to all thoughtful persons a moral demonstration that the Christian religion is not of men, but of God."[21]

It is in disunity that God's church is most helpless in the present times. Nothing is more productive of infidelity and unrighteousness than the conflicting doctrines of professed followers of Christ. By multiplying divisions, Satan has hindered numberless millions from obeying the gospel. No greater need could be imagined than that of the unity of the church of the living God; but alas, only a certain kind of unity will avail anything; and that is the kind of unity Jesus identified in this prayer, a unity like that between the Father and the Son.

Satan has ever been busy advocating his own kind of unity, such as: (1) the unity of authoritarianism, in which all blindly obey the ecclesiastics elevated above them; (2) the kind of unity proposed by the snake to the frog, in which one entity is swallowed up in another; (3) the unity in which each group of believers accepts his status under some system of allocation, and in which, like in the cemetery, everyone lies as complacently as possible and does not infringe on his neighbors; (4) the unity in which many groups are submerged in a super-organization, thus containing every degree of contradiction and aberration under one pretentious banner, such unity being very similar to that exhibited by a barrel of scorpions.

Therefore,

Believers should always yearn for peace, but never for peace at the expense of truth; for "unity" which has been gained by means of such a sacrifice is not worthy of the name.[22]

Thou Father art in me, and I in thee ... and they in us ... This threefold unity is the only kind of unity that can avail. For notes on the profound implications of being "in Christ," see under John 14:20. To be "in Christ" is also to be "in God."

[21] Robert Milligan, Analysis of the New Testament (Cincinnati, Ohio: Bosworth, Chase, and Hall, 1874), p. 268.

[22] William Hendriksen, op. cit., II, p. 365.