John Calvin Commentary Titus 1:16

"They profess that they know God; but by their works they deny him, being abominable, and disobedient, and unto every good work reprobate." — Titus 1:16 (ASV)

They profess that they know God. He treats those persons as they deserve; for hypocrites, who give their whole attention to minute observances, despise fearlessly what constitutes the chief part of the Christian life. The consequence is that they display their vanity, while contempt for God is manifested in open crimes. And this is what Paul means: those who wish to be seen abstaining from one kind of food indulge in wantonness and rebellion, as if they had shaken off the yoke. Their conduct is disgraceful and full of wickedness, and not a spark of virtue is visible in their whole life.

For they are abominable, disobedient, and to every good work reprobate. When he calls them, βδελυκτούς, abominable, he seems to allude to their pretended holiness, to which they gave their earnest attention. But Paul declares that they gain no advantage, for they do not cease to be profane and detestable. With good reason he accuses them of disobedience. For nothing can be more haughty than hypocrites, who work so laboriously on ceremonies so that they can despise with impunity the chief requirements of the law. We may appropriately interpret the word ἀδόκιμοι, reprobate, in an active sense; it is as if he had said that those who wish to be thought such sagacious instructors in trifles are destitute of judgment and understanding regarding good works.