John Calvin Commentary Jude 1:20

"But ye, beloved, building up yourselves on your most holy faith, praying in the Holy Spirit," — Jude 1:20 (ASV)

But ye, beloved. He shows the way they could overcome all the devices of Satan: that is, by having love connected with faith and by standing on their guard, as it were, in their watchtower, until the coming of Christ. But just as he often and densely uses his metaphors, so he has here a way of speaking unique to him, which must be briefly noted.

He instructs them first to build themselves on faith; by which he means that the foundation of faith should be retained, but that initial instruction is not sufficient unless those who have already been grounded in true faith continually progress towards perfection. He calls their faith most holy, so that they may wholly rely on it and, leaning on its firmness, may never vacillate.

But since the whole perfection of man consists in faith, it may seem strange that he instructs them to build upon it another building, as though faith were only a beginning for man. This difficulty is removed by the Apostle in the words that follow, when he adds that men build on faith when love is added; unless, perhaps, someone may prefer to understand it this way: that men build on faith insofar as they advance in it. And doubtless, the daily progress of faith is such that faith itself rises up as a building. Thus the Apostle teaches us that in order to increase in faith, we must be persistent in prayer and maintain our calling by love.

Praying in the Holy Ghost. The way to persevere is to be endowed with the power of God. Hence, whenever the constancy of faith is in question, we must flee to prayer. And as we commonly pray in a formal manner, he adds, In the Spirit; as though he had said that such is our sloth, and such is the coldness of our flesh, that no one can pray correctly unless roused by the Spirit of God; and that we are also so inclined to lack of confidence and trembling, that no one dares to call God his Father except through the teaching of the same Spirit. For from him is earnest care, from him is passion and intensity, from him is eagerness, from him is confidence in obtaining what we ask; in short, from him are those unutterable groanings mentioned by Paul (Romans 8:26). It is not, then, without reason that Jude teaches us that no one can pray as he should without having the Spirit as his guide.