Of what fact was Paul convinced in the Lord Jesus?...

That nothing is unclean in itself.

Romans 14:14, like every other passage of Scripture, must be interpreted in terms of the whole Word of God.

Sometimes this verse is used to justify something that indeed is unclean of itself. Fornication, lust, stealing, lying, covetousness, foolishness, rage, drunkenness, envying, profanity are unclean of themselves. They are the expressions of demons.

Romans 14:14 is a verse that the ungodly can wrest to their own destruction.

God has left a door open here so the wilfully filthy person, the person who chooses to be ignorant and unclean, can deceive himself.

God always examines the hearts of the people on the earth. The person who is hungering and thirsting after righteousness will be filled with righteousness. The person who delights in evil will find many opportunities to achieve the desires of his heart and will be able to justify his filthiness from the Scriptures. God catches the crafty in their craftiness.

We always must guard our heart with all diligence, for from it flow the issues of life. Our speech, meanwhile, either justifies or condemns us in the sight of God.