Theophylact of Bulgaria
By these words he confirms two points. First, the one who is conquered necessarily serves his conqueror. Second, those who have known the truth but again hold to their former impiety fall into an evil worse than before. He adds a proverb that confirms the same thing. See Prov 26:11. The meaning is this: if those who, through the knowledge of the Lord and Savior Jesus Christ, escaped the defilements of the world are again entangled in them and conquered by them, then without doubt they are enslaved by them. Their condition becomes worse than it was before they knew of bondage, for Satan strives to make them fall into greater evil.