For certain people have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ.Here is the first focus of Jude's writing. He has noticed that there are people who have become a part of the church, that are perverting the Gospel. This perversion is two-fold: First they are abusing God's grace as an excuse to live out their lives according to their lusts. Grace abounds, of course, so they have permission to continue to sin. So, instead of denying self and taking up the cross, they are claiming Christ without sacrificing their lusts.
This leads to the second perversion, which is the denial of Jesus Christ as the Only Master and Lord. When you live by your lusts, you are denying the Lordship of Christ.