“When [Jesus] adds thine they were, and thou gavest them to me [Jn 17:6], He first indicates the eternity of election, and then how we should think of it. Christ declares that the elect always belong to God. God therefore distinguishes them from the reprobate, not by faith, nor by any merit, but by pure grace; for while they are completely alien to Him, He yet regards them as His own in HIs secret counsels. The certainty consists in His committing to the guardianship of HIs Son all whom He has elected, that they may not perish; and it is there that we must turn our eyes if we are to be certain that we are of the number of God’s children. For in itself the predestination of God is hidden; and it is manifested to us in Christ alone.”
John Calvin, Commentary on the Gospel According to St. John: Part Two, Transl. by T.H.L. Parker, p. 139.