That Satan is at work in the fallen people (everyone of us) is shown in Jesus rebuking Satan behind the words of His leading disciple -
Peter. In Peter's demand the Jesus pity Himself and not go to the cross, the wise Son of God detected Satan's work to hinder the operation of God to save man.
From that time Jesus began to show to His disciples that He mist go to Jerusalem and suffer many things from the elders and chief priests and scribes and be killed and on the third day be raised.
And Peter took Him aside and began to rebuke Him [Jesus], saying, God be merciful to You, Lord! This [crucifixion] shall by no means happen to You!
But He [Jesus] turned and said to Peter, Get behind Me, Satan! You are a stumbling block to Me, for you are not setting your mind on the things of God, but on the things of men." (Matt. 16:21-23)
You see, hiding out in the self of fallen sinners is the enemy of God wanting to KEEP man in bondage AND to keep the earth for Satan.
Getting saved is not only a step to rescue sinners from judgment. It is also a strategic step to teach man how to detect God's enemy and DEFEAT Him by denying the fallen self and following the indwelling Spirit of Christ.
Following this rebuke of Satan behind Peter, Jesus says to follow Him we must learn to enjoy Him by denying our selves, our fallen devil infiltrated nature.
" ... you are not setting your mind on the things of God, but on the things of men.
Then Jesus said to His disciples, If anyone wants to come after Me, let him deny himself and take up his cross and follow Me. For whoever wants to save his soul life shall lose it; but whoever loses his soul-life for my sake shall find it.
For what shall a man be profitted if he gains teh whole world, but forfeits his soul-life? or what shall a man give in exchange for his soul-life?
For the Son of Man is to come in the glory of His Father with His angels, and He will repay each man according to His doings." (Matt. 16:23b-27)
So Witness Lee was showing the audience what to conquer and how to conquer. It is mainly by denying the self, even the religious self, and enjoying the reality of the indwelling Christ.
The enemy is stopped by the cross. The Christian "carrying" his cross goes in victory in resurrection life.
Lee went on -
We may feel that as Christians, we should conquer sin, the world, our flesh, and our self. But Revelation 2 and 3 show that the Lord wants us to conquer three things: Judaism, Catholicism, and Protestantism.
I will devote another post to this. But what he is saying briefly is:
The Christian may think it is just a matter of him spiritually overcoming the weaknesses of the flesh, the pull of the world, the fallen self, etc.
But in actuality the spiritual warfare is over the "ISMS" which have kept God's people befuddled in religions of various kinds.
On one hand most religions seem as noble things in human culture. But the subtlety of God's enemy often is in religion to keep people in something other than the living Christ, the living God, and the living Spirit to win the battle over the earth.
We must fight for the experience of the Triune God and not be dampened in our living by ISMS of organized religion.
One of the main books by Witness Lee which spoke to this issue was
Christ Verses Religion.