(20, 21) Exhortation to strengthen themselves in the faith by prayer, godliness, and hope.
(20) But ye, beloved. — Exactly as in Jude 1:17 : “ye” in emphatic contrast to these sensuous and unspiritual men.
Building up yourselves. — Making yourselves firm on the sure foundation of faith, in contradistinction to those “who separate,” and fancy themselves firm in their impious conceits. The notion is not so much that of increasing and completing an edifice as of strengthening its foundations. Faith and its object are here almost identified. To have faith as one’s foundation is the same as having Christ as one’s foundation. “Your faith,” that which has been “once for all delivered” to you (Jude 1:4). “Most holy faith,” as opposed to the most unholy quick sands of the doctrines condemned in this Epistle.
Praying in the Holy Ghost. — Only in this way can Christians make firm their foundation. The Greek admits of “in the Holy Ghost” being taken with the previous clause; but our version is better. The expression “praying in the Holy Ghost” is not found elsewhere. It means that we pray in His strength and wisdom: He moves our hearts and directs our petitions. (See Notes on Romans 8:26.)
(21) Keep yourselves in the love of God. — Not our love of God, but His love of us. Consequently it is not the case that the three great Christian virtues — Faith, Hope, and Charity — are inculcated here, although at first sight we are tempted to think so. God’s love is the region in which those who are built up on faith, and supported by prayer, may continually dwell.
The mercy of our Lord Jesus Christ. — The mercy which He will show as Judge at the Last Day. By prayer in the Spirit we are kept in the love of the Father for the mercy of the Son.
Unto eternal life. — These words may be taken either with “keep yourselves,” or with “looking,” or with “mercy”: best with “keep yourselves.”