They profess that they know God; but in works they deny him, being abominable, and disobedient, and unto every good work reprobate.
They profess - i:e., make a profession acknowledging God. They have theoretical knowledge of God, but practically know Him not.
Deny him - the opposite of the previous "profess" or 'confess' Him (1 Timothy 5:8; 2 Timothy 2:12; 2 Timothy 3:5). So often as we are conquered by sins, we deny God (Jerome).
Abominable - themselves, though laying such stress on avoiding abomination from outward things (cf. Leviticus 11:10-13; Romans 2:22).
Disobedient - to God (Titus 3:3; Ephesians 2:2; Ephesians 5:6). Reprobate - rejected as worthless when tested (notes, Romans 1:28; 1 Corinthians 9:27; 2 Timothy 3:8).