"But ye, beloved, building up yourselves on your most holy faith, praying in the Holy Ghost, Keep yourselves in the love of God, looking for the mercy of our Lord Jesus Christ unto eternal life" (Jude 20-21).
The "beloved," whom the apostle Jude addresses here at the end of his epistle, were the same "beloved" to whom he was writing at its beginning (v.3). There, he had urged them to "earnestly contend for the faith"; here, he urges them to "build themselves up in the faith." The next two verses stress Christian witnessing: "And of some have compassion, making a difference: And others save with fear, pulling them out of the fire" (vv.22-23). There is, therefore, no conflict between contending for the faith, growing in the faith, and seeking to win converts to the faith. Some Christians would argue for one or the other of these occupations as highest priority in the Christian life, but all are vital.
Most of Jude's epistle (vv.3-19) is taken up with an urgent exhortation to defend the Christian faith against its enemies, especially those who seek to subvert it from within the Christian community itself. Except for the concluding doxology (vv.24-25), it then ends with a call to win souls for Christ, using whatever means are effective with each particular individual, whether love or fear or both. But the two verses of our text would imply that neither our efforts in apologetics nor those in evangelism can be very productive without simultaneously attending to the stabilizing and strengthening of our own personal faith in Christ and His word. Peter similarly concludes his writings (II Peter 3:18).
But how do we grow in grace and knowledge, and how do we build ourselves up in the faith? Peter says to stir our minds with the Holy Scriptures (II Peter 3:1-2), and Jude says to pray in the Holy Ghost (Jude 20). Study, pray, and then obey--day after day--that's the way! HMM