Keep yourselves in the love of God, looking for the mercy of our Lord Jesus Christ unto eternal life.
In Jude 1:20-21, Jude combines the Father, Son, and Holy Spirit: faith, hope, and love.
Keep yourselves - not in your own strength, but "in the love of God;" i:e., God's love to you and all believers: the only guarantee for their being kept safe. Man needs to watch; but cannot keep himself, unless God in His love keep him.
Looking for - in hope.
The mercy of our Lord Jesus Christ - to be fully manifested at His coming. Mercy is usually attributed to the Father: here to the Son; so entirely one are they.