These men were not denying Jesus Christ in a way that made it obvious, otherwise they would not have been welcomed into the Christian community nor listened to. They were thus busy pretending allegiance to Jesus Christ, though their actions would show who was truly their master.
4 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.