They profess to know God, but by their deeds they deny Him, being detestable and disobedient and worthless for any good deed. (Titus 1:16)
Paul is still referring to the Jews mentioned earlier. They knew OF God. Their fathers and their fathers before them knew OF God. But that knowledge was clinical and impersonal. They did not KNOW God.
Paul then goes on to say that despite the profession of their mouths that they know God, their practices betray what was really believed. But its worse than that. Their actions actually deny God.
These folks were practical atheists. While their mouths claimed that they knew God, their very works, their very actions said “there is no God”.
In the sight of God, their actions were detestable, idolatrous. They had replaced God with process. They had replaced God with methods. They had replaced God with tradition.
If you don’t really believe there is a God, then you don’t really have to follow His laws. The secret desires of the heart blossom into public disobedience. Church members refuse to acknowledge Matthew 18 style church discipline. Teachers teach whatever comes to them instead of God’s word. Members refuse to gather faithfully in fellowship, content to spend the weekends lounging.
Finally Paul points out, that no one should expect anything good to come from these folks. How can a teacher that refuses to acknowledge the authority of scripture be expected to yield Godly advice to students? How can a deacon who sees no problem with divorce and remarriage in the church be expected to defend and protect the Bride of Christ from evil from without? When the advice given to students is always from personal experience and never from the Word, what kind of outcome will that yield?