Jamieson, Fausset and Brown on Titus 1:16

16. They profess--that is, make a profession acknowledging God. He does not deny their theoretical knowledge of God, but that they practically know Him.

deny him--the opposite of the previous "profess" or "confess" Him (1Ti 5:8; 2Ti 2:12; 3:5).

abominable--themselves, though laying so much stress on the contracting of abomination from outward things (compare Le 11:10-13; Ro 2:22).

disobedient--to God (Tit 3:3; Ep 2:2; 5:6).

reprobate--rejected as worthless when tested (see on Ro 1:28; 1Co 9:27; 2Ti 3:8).