John Calvin Commentary Jude 1:21

"keep yourselves in the love of God, looking for the mercy of our Lord Jesus Christ unto eternal life." — Jude 1:21 (ASV)

Keep yourselves in the love of God. He has made love, so to speak, the guardian and the ruler of our life. This is not because He would set it in opposition to the grace of God, but because it is the right course of our calling when we make progress in love.

However, since many things entice us to apostasy, so that it is difficult to remain faithful to God until the end, he directs the attention of the faithful to the last day. For the hope of that day alone should sustain us, so that we may never despair; otherwise, we would inevitably fail every moment.

But it should be noted that he would not have us hope for eternal life, except through the mercy of Christ: for He will in such a way be our judge, as to have no other rule in judging us than that gratuitous benefit of redemption which He Himself obtained.