Starting Point

Contextual summary:

Context: 2 Peter 2:17-22
These verses describe “false prophets” and “false teachers”; note that the false prophets are among the people and the false teachers are among you, referring to believers. These false ones introduce false teachings with the result of leading people astray. The ones lead astray include believers (those who barely escape). Believers who return to their own “vomit” are worse off than those who never new Christ.

2 Peter 2:20

This verse is speaking directly to believers, we understand this because “they… escaped by the knowledge of… Christ.”
This verse contains two thoughts, the main thought and a parenthetical thought. The main thought is contained in the words, “For if they are again entangled in them and are overcome, the last state has become worse for them than the first”. And the parenthetical thought is “after they have escaped the defilements of the world by the knowledge of the Lord and Savior Jesus Christ”. We shall consider the parenthetical thought first as it must be understood in order to fully comprehend the main thought.
Notice that “they” had escaped. “They” had avoided by means of fleeing from the defilements and, for whatever reason, “they” stopped actively fleeing; by implication “they” also stopped pursuing the knowledge of Jesus Christ.
Salvation has many different aspects. When we believe in Christ we are saved there and then eternally. "and after he brought them out, he said, “Sirs, what must I do to be saved?” They said, “Believe in the Lord Jesus, and you will be saved, you and your household.”" (Acts 16:30-31, NASB95)
We are saved daily from the control of sin by the means of bible study. "Be diligent to present yourself approved to God as a workman who does not need to be ashamed, accurately handling the word of truth." (2 Timothy 2:15, NASB95)
Ultimately we will be saved from the presence of sin “Then I saw a new heaven and a new earth; for the first heaven and the first earth passed away, and there is no longer any sea." (Revelation 21:1, NASB95)
Once a person becomes a believer in Christ they maintain their escape from defilements by seeking the knowledge of Christ. This process is outlined in Colossians 3:1-3.
“Therefore if you have been raised up with Christ, keep seeking the things above, where Christ is, seated at the right hand of God. Set your mind on the things above, not on the things that are on earth. For you have died and your life is hidden with Christ in God." (Colossians 3:1-3, NASB95)
Note that you are to set your mind on the things above. This is a conscious decision to seek Christ. This is accomplished by daily prayer and bible study. This is not a list of do’s and don’ts it is however a slow change from the old to the new. Any attempt to stop doing “worldly” things will fail unless that stopping is the result of “worldly things” being replaced with the “things of God”. As a person grows in Christ the things of the world will decrease in importance.