John Calvin Commentary Jude 1:24

"Now unto him that is able to guard you from stumbling, and to set you before the presence of his glory without blemish in exceeding joy," — Jude 1:24 (ASV)

Now unto him that is able to keep you. He closes the Epistle with praise to God, by which he shows that our exhortations and labors can do nothing except through the power of God accompanying them.

Some copies have “them” instead of “you.” If we accept this reading, the sense will be, “It is, indeed, your duty to endeavor to save them; but it is God alone who can do this.” However, the other reading is what I prefer, in which there is an allusion to the preceding verse.

For after having exhorted the faithful to save what was perishing, so that they might understand that all their efforts would be vain unless God worked with them, he testifies that they could not be saved except through the power of God.

In the latter clause, there is indeed a different verb, φυλάξαι, which means to guard; so the allusion is to an earlier clause, when he said, Keep yourselves.