Keep Yourselves in the Love of God

Jude calls upon the saints to persevere, but he uses the phrase: 'Keep yourselves in the love of God.' What does that mean? Can Christians ever fall outside the love of God? Why does God warn His people of apostasy if none of His people will ever fall away? Jude describes what perseverance looks like: (1) Building ourselves up on our most holy faith, (2) praying in the Spirit, and (3) looking for the mercy of our Lord Jesus Christ unto eternal life.