John 14:20

In that day, when I am risen again, or when the Holy Ghost is come, you shall know that I am in the Father, and how, and in what manner: as also, how I am in you, and you in me. Our Saviour Christ, on several occasions, speaks of different ways of being united, or of being one; as first, of being one in nature and substance, and by such an union, as agrees only with the divine persons, who are one in all things. 2ndly, Persons may be one, or united in affection and love, which also, as to its most perfect manner, agrees only with the three divine persons; but a similitude, and an imitation of this union of love, is found among creatures, both when they love God, and when for God's sake, they love one another: yet these unions are as different as God, and his creatures. The Arians and Socinians lay hold on these expressions, and of the words, (chap. xvii. ver. 21.) when Christ prays, that his disciples may be one, as he and his Father are one, which words imply no more than a similitude, and an imitation of that union of love (with which the three divine persons love one another) though at an infinite distance. If the old or new Arians examined, with a sincere desire of finding the truth (which they ought to seek from many passages in the New Testament, as well as from the sense and tradition of the Church, guided by the promised Spirit of Truth) they might certainly find how different is the union of nature and substance of the eternal Son with his eternal Father, and of that union of the three divine persons, when they are said to be one, from that inferior and lesser union of love and affection, by which either God loves his adoptive children, his faithful servants, or they love one another: they would easily discover, that many things are said of the unity and union of the divine persons, which could not be true, unless they were one and the same God, coeternal and consubstantial, which by no means can be said of God and his creatures, nor of the union of affection only, by which the creatures love one another. (Witham)