Jude 20–21 (ESV): But you, beloved, building yourselves up in your most holy faith and praying in the Holy Spirit, keep yourselves in the love of God, waiting for the mercy of our Lord Jesus Christ that leads to eternal life.
Jude 20 (MSB): building. True believers have a sure foundation (1 Cor. 3:11) and cornerstone (Eph. 2:20) in Jesus Christ. The truths of the Christian faith (cf. v. 3) have been provided in the teaching of the apostles and prophets (Eph. 2:20), so that Christians can build themselves up by the Word of God (Acts 20:32). praying in the Holy Spirit. See note on Eph 6:18. This is not a call to some ecstatic form of prayer, but simply a call to pray consistently in the will and power of the Spirit, as one would pray in the name of Jesus Christ (cf. Rom. 8:26, 27).