John 17:21
“That they all may be one, as You, Father, are in Me and I in You.”
Here again the “as” does not denote exact similarity in their case, (for it was not possible for them in so great a degree,) but only as far as was possible for men. Just as when He says “Be merciful, as your Father.”
But what is, “In Us”? In the faith which is on Us. Because nothing so offends all men as divisions, He provides that they should be one. “What then,” says some one, “did He effect this?” Certainly He effected it. For all who believe through the Apostles are one, though some from among them were torn away. Nor did this escape His knowledge, He even foretold it, and showed that it proceeded from men's slack-mindedness.
“That the world may believe that You have sent Me.”
As He said in the beginning, “By this shall all men know that you are My disciples, if you love one another.” And how should they hence believe? “Because,” He says, “You are a God of peace.” If therefore they observe the same as those of whom they have learned, their hearers shall know the teacher by the disciples, but if they quarrel, men shall deny that they are the disciples of a God of peace, and will not allow that I, not being peaceable, have been sent from You. Do you see how, unto the end, He proves His unanimity with the Father?
Source: Homilies on the Gospel of John (New Advent)