New Defender's Study Bible Notes
20 building up yourselves. Note that it is perfectly consistent for Christians to be building themselves up in the faith, having a strong prayer life, and witnessing to others (Jude 22,23) while at the same time contending earnestly for the faith (Jude 3). Apologetics and Christian evidences are mutually compatible with evangelism and Christian growth.
21 eternal life. This is the last of forty-one New Testament occurrences of the phrase “eternal life” or “everlasting life.” The first was in Matthew 19:16, and the central one John 10:28. Thus:
“What good thing shall I do, that I may have eternal life?” (Matthew 19:16).
“I give unto them eternal life; and they shall never perish” (John 10:28).
“Keep yourselves in the love of God, looking for the mercy of our Lord Jesus Christ unto eternal life” (Jude 21).
23 save with fear. Our witnessing should normally be with “compassion” (Jude 22), but sometimes, depending on circumstances, with sober warnings of hell. It should, of course, always be Biblical, truthful, and done in loving concern.