"But ye, beloved, building up yourselves on your most holy faith, praying in the Holy Spirit, keep yourselves in the love of God, looking for the mercy of our Lord Jesus Christ unto eternal life." As in ver. 17, the "But _ye_, beloved" (ὑμεῖς δέ, ἀγαπητοί) makes an emphatic contrast between those whom St. Jude addresses and the sensuous and unspiritual men of whom he has been speaking. He exhorts his readers to endeavour to keep themselves in favour with God by cultivating faith, prayer, and hope; and in this exhortation the main purpose of the letter, as set forth in ver. 3, is fulfilled. The triplet of participles (ἐποικοδομοῦντες--προσευχόμενοι--προσδεχόμενοι) must not be lost sight of, although the fact that the main verb (τηρήσατε) comes in the middle of them, instead of at the end, somewhat obscures the triple construction.
The expression "building up" (ἐποικοδομεῖν) is in the New Testament never used of actual building, but always in the metaphorical sense of believers being united together so as to form a temple. In this temple Christ is sometimes regarded as the foundation (1 Cor. iii. 11), sometimes as that which binds the structure together (Eph. ii. 20; Col. ii. 7). The notion of building up comes from the preposition (ἐπί), one stone being placed upon another, so that upward progress is made. "The faith" here is probably the foundation on which the structure is to rest; but it would be possible to translate "_with_ your most holy faith," instead of "_on_ your most holy faith;" and in that case the dative would, as in Col. ii. 7, express the cement rather than the foundation. In any case "the faith" is not the internal grace or virtue of faith, but, as both the participle and the adjective show, "the faith which was once for all delivered unto the saints" (ver. 3). It is "_your_ faith," because it has been thus delivered to you; and it is "most holy," in marked contrast to the vile and shifty doctrines which the libertines profess and uphold.
"Praying in the Holy Ghost." This is the best arrangement of the words, although the Greek allows us to take "in the Holy Ghost" with the previous clause, a rather clumsy division of the words, which is sanctioned by Luther, Beza, and the Rhemish Version: "building yourselves upon our (_sic_) most holy faith, in the Holy Ghost, praying." The expression "praying in the Holy Ghost" occurs nowhere else; but that is no reason why St. Jude should not have used it here. It means that we are to pray in the power and wisdom of the Spirit. In order that we may pray, and pray aright, He must move our hearts and direct our petitions.
"Keep yourselves in the love of God." Not our love of God is meant, but His love of us. This is rendered probable both by what immediately follows--for "the love of God" should have a meaning similar to that of "the mercy of Jesus Christ"--and also by the opening address, "beloved in God" (ver. 1), which St. Jude perhaps has in his mind; for the whole of the verse before us is closely connected with the first verse of the Epistle. God's love is the region in which all Christians should strive to abide, and it is by faith and prayer that this abode is secured. To be conscious of being beloved by God is one of the greatest protections that the believer can possess.
"Looking for the mercy of our Lord Jesus Christ unto eternal life." That mercy which He will show to all faithful Christians when He returns as Judge at the last day. We may compare "looking for and earnestly desiring the coming of the day of God" (2 Peter iii. 12). Both in this life and in eternity it is mercy that we need and crave. The Psalms are full of this thought, as a reference to the numerous passages in which the word mercy occurs will reveal: see especially Ps. cxxx. And in connexion with this the concise statement respecting the relations of the Persons of the Blessed Trinity to believers must not be overlooked. By prayer in the power of the Holy Spirit we are kept in the love of the Father through the mercy of the Son. "Unto eternal life." It is not a matter of much moment whether we take these words with "keep yourselves," or with "looking," or with "mercy." The first seems to be the best arrangement, "keep yourselves ... unto eternal life;" but in any case the eternal life is reached through the mercy of the Lord Jesus Christ. With a similar thought the author of the Epistle to the Hebrews (ix. 28) writes of Christ's Second Advent as an advent "unto salvation" (εἰς σωτηρίαν). The Divine purpose of both Advents is mercy, and not judgment; but seeing that both Advents are met by some who refuse to believe and repent, judgment is inevitable.