Jude 21a says, "Keep yourselves in the love of God." The love of God is produced and shed abroad in our hearts by the Holy Spirit (Romans 5:5). Still, the Word of God exhorts us to keep ourselves in that love.
What does it mean to "keep yourselves"? The word "keep" is tereo in the Greek and it means to watch over, to preserve, and to keep. Believers keep themselves by understanding and using positive volition to mature as they advance into greater grace. The prerequisite to keeping ourselves in that love is to have an expectation of mercy from the Lord.
"But ye, beloved, building up yourselves on your most holy faith, praying in the Holy [Spirit]" (Jude 20). "Beloved," agapetos, refers to those who are dearly beloved of God because of the grace of Calvary. This passage is written in an imperative mood, giving a divine command to pray and to build up ourselves continually.
As the beloved of God, we are commanded to watch over, guard, preserve, inspect, and keep our own souls through positive volition. That way, we can keep ourselves continually in the power of the Holy Spirit, who brings in the love that is shed abroad in our hearts.