Why is Satan constantly attacking the Church?
Satan constantly attacks the Church because he knows that in the Church realm he has his greatest opposition. If he can neutralize the Body of Christ so it says nothing, he can have full sway, corrupting minds from the simplicity that is in Christ, just as the serpent beguiled Eve.
Les explains from II Corinthians 11:3 that the serpent beguiled Eve through subtlety, and so the minds of believers can be corrupted from the simplicity that is in Christ. If Eve is a symbol of the Church, the Body of Christ, then it stands to reason that Satan constantly attacks the Church. Les says this is because Satan knows it is in the realm of the Church that he has his greatest opposition. Les also notes the saying that when good men do nothing, bad things happen; Satan knows that if he can neutralize the Body of Christ so it says nothing, he can have full sway. Verse 4 further warns of another Jesus, another spirit, and another gospel that they might bear well with him.
But I fear, lest by any means, as the serpent beguiled Eve through his subtilty, so your minds should be corrupted from the simplicity that is in Christ.