John 17:21

That they all may be one,.... Among themselves. This may regard their unity in faith, and in the knowledge of Christ; for there is but one faith which all truly experienced souls agree in: they are converted by the same Spirit, and have the same work of grace wrought in them; and though they have not the same degree of light, and measure of spiritual knowledge, yet they agree in the main point of the Gospel, salvation alone by the Lord Jesus Christ; and such an agreement in the doctrine of faith, and the grand articles of it, respecting the person and offices of Christ, is absolutely necessary to their comfortably walking together in church fellowship, and the more honourable carrying on the worship of God together; and which will be more manifest in the latter day, when saints shall see eye to eye: likewise an unity in affection may be here designed, a being knit together in love to each other; which is the bond of perfectness, the evidence of regeneration, the badge of the Christian profession, the beauty of church communion, and the barrier and security from the common enemy; and without which, social worship and mutual service will be either neglected or coldly performed. Moreover, this petition may have respect to the gathering together all the saints at the last day, as one body united together in faith and love; as one general assembly and church of the firstborn; as one fold of which Christ is the head, Saviour, and shepherd:

as thou, Father, [art] in me, and I in thee. There is a mutual in being of the Father and the Son, who are one in nature and essence, in power and will, and in understanding and affection; which union, though it infinitely transcends any kind of union among men, or that can be conceived of by men, yet is the exemplar of the saints' union one with another, and to the divine persons; and which must be understood not of an equality, but a likeness:

that they also may be one in us: there is an union of all the elect to God and Christ, which is already complete, and not to be prayed for; they are all loved by God with an everlasting love, by which they are inseparably one with him; they are all chosen in Christ, as members in their head, and are federally united to him, as their Mediator, surety, and representative; in consequence of which he has assumed their nature, stood in their place and stead, and brought them nigh to God. There is a manifestation of union in conversion, when persons openly appear to be in Christ; and as a fruit and effect of everlasting love, are with loving kindness drawn unto him; and which will be more gloriously seen, when all the elect shall be brought in, and God shall be all in all, and is what Christ here prays for:

that the world may believe that thou hast sent me; either the rest of God's chosen people in the world, not yet called; or rather the wicked and reprobate part of the world, particularly Jews and Deists: they shall see the concord and agreement of the saints in doctrine, worship, and affection in the latter day; and when all the elect shall be gathered together, and not only their union to each other, but to the divine persons, shall clearly appear; they will then believe, ant be obliged to own, that Jesus is the true Messiah, was sent of God, and is no imposter.