A.T. Robertson Commentary 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 in us: that the world may believe that thou didst send me." — John 17:21 (ASV)

That they also may be in us (ινα κα αυτο εν ημιν ωσιν). Another purpose clause with ινα and the present active subjunctive of ειμ. The only possible way to have unity among believers is for all of them to find unity first with God in Christ.

That the world may believe (ινα ο κοσμος πιστευη). Another purpose clause with ινα and the present active subjunctive of πιστευω, "may keep on believing." Beyond a doubt, strife, wrangling, division are a stumblingblock to the outside world.