The key to grasping this austere regimen lies in the phrase “basic principles of the world” (verse 20; “rudiments” in the KJV). The subject of Paul's teaching does not involve God's laws at all but worldly, pagan teachings that involve asceticism and demon worship. A “rudiment” is a basic, elementary principle or act of worship, and these rudiments are drawn from the world. These ascetic practices have nothing to do with God's true religion. Verse 22 confirms this when Paul writes that these regulations are the decrees and teachings of men, not God.
Paul's counsel on the extreme disciplines of the super-righteous, such as those practiced in the world by ascetics, is that they produce a puffed-up mind—pride, a haughty spirit—rather than humble obedience that truly impresses God, such as that praised so highly in Isaiah 66:1-2:
“Heaven is My throne, and earth is My footstool. Where is the house that you will build Me? And where is the place of my rest? For all those things My hand has made, and all those things exist,” says the Lord. “But on this one will I look; on him who is poor and of a contrite spirit, and who trembles at My word.”
In no way is the apostle teaching that we must not discipline ourselves to live balanced lives within God's laws to avoid sin.
John W. Ritenbaugh
Ecclesiastes and Christian Living (Part Twelve): Paradox, Conclusion