David delineates a significant difference between the godly and ungodly. The distinction lies in the way and how often each thinks about God. The fundamental differences are in how important God is to each and how accurate their thoughts about Him are.
In the Millennium, when God's Kingdom is reigning on earth, the priesthood will teach the difference between clean and unclean! And after the thousand years, no abominable thing, nothing that defiles will mar the New Jerusalem! All of its citizens will be holy. This is the wonderful destiny that we are preparing for, and part of making ourselves ready is following the law of clean and unclean meats.
As the spiritual temple of God, as God's sons and daughters, we have a duty to make ourselves as "clean," "pure," or "holy" as possible. This "perfecting holiness" includes all areas of life, not just the spiritual. Paul makes an unmistakable distinction between flesh and spirit (II Corinthians 7:1) only two verses after he paraphrases Isaiah 52:11: "Do not touch what is unclean."