Corruption agrees with unbelief
Some people have wrongly used these verses to justify wicked behaviour. They say, ‘Something is only unclean if you think of it that way; if you think it is beautiful then enjoy God's gift.’ But by saying that, they prove how corrupted they are. If God's Word says something is wrong, then it is wrong ... it is what God says that matters, not what we think (Jeremiah 7:4-10). If we disobey God, what we do can never be beautiful or pure.
Paul had just instructed Titus to confront those who had been fooled by the false teachers and become counterfeit Christians (Titus 1:14). They believed they were pure because they kept some religious rules, but they did not know Jesus (Matthew 7:21-23). Their unbelief distorted their vision about themselves and God. Consequently, they started to think that right was wrong and wrong was right. Even their consciences became spoiled so that they had nothing to stop them descending further into wickedness (1 Timothy 4:2). Even though they had 'joined the church', their disobedience became pruriently offensive to God (Revelation 3:16-17); they were not fit for God's purpose.
To describe people as “… detestable, disobedient and unfit for doing anything good …” may seem very harsh, but Paul knew that fake Christians would ruin the church's ability to grow or even exist. Such people are still one reason why churches lose their credibility, vitality and spiritual connection with Jesus. Tolerating them out of a misguided sense of niceness, assuming that they cannot do any harm, is the fast route to corrupting the church (Revelation 2:14-16; Revelation 2:20-21).
Paul's instructions to Titus continue to speak to us. 1. Rebuke the ringleaders (Titus 1:11-12); 2. Teach the apostolic gospel (Titus 2:1, Titus 2:15); 3. Encourage those who will listen; 4. Rebuke those who will not (Titus 3:9-11). In the same way that one bad fruit will infect many others, so one fake Christian may lead others astray. So, pray for your spiritual leaders that they will have great confidence in God's Word, skill in teaching the truth and refuting error, courage to challenge what is wrong and gracious love to persist despite difficulty.