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In verse 4, Jude gives a few general points about false teachers. The literal translation of the first part is, "For certain men have wormed their way in." They slithered in like snakes without others being aware, on the sly. God, however, marked them out as enemies a long time ago; He knows they are there. This agrees with Jesus' Parable of the Wheat and the Tares (Matthew 13:1). There, God allowed the adversary to sow the tares among the wheat, but He left them in the field for a reason. Paul says in I Corinthians 11:19 that they are left to show "who are approved" and who is not. Over time, God's enemies are shown in stark contrast to His true children.
Jude calls these false teachers "ungodly" or impious. The word "ungodly" crops up six times in the book of Jude, making it a significant word in the epistle. It means "irreverent" or "impious." People who are ungodly are not afraid to contravene God's way because they do not have the fear of God. They simply do what they feel like doing. They have nothing in common with God. Nevertheless, we should be careful not to think of them as "extreme" because, as II Corinthians 11:13-15 says, Satan transforms his ministers into angels of light. They look good on the outside, but on the inside they are ungodly. Their externals may be deceiving, but their fruits give them away.
The apostle also mentions that they are blatantly immoral, turning God's grace into licentiousness. They believe that the more they sin, the more they allow God to shower them with His grace and mercy, twisting what grace is. They think they are glorifying God by giving Him the opportunity to forgive them more. How ridiculous! How perverse!
Lastly, they deny Christ. This does not mean they say that Christ never existed or that He is not the Savior. However, everything they say and do, everything they believe, contradicts God's way. If one denies a statement, he is contradicting the person who says it. Jude is using "deny" in this sense. The false teachers contradict Jesus Christ in everything. Once again, they can appear to be following the rules, but their innermost drives and motivations are a denial of the true way of God.
Richard T. Ritenbaugh