19 For your obedience has become known to all. Therefore I am glad on your behalf; but I want you to be wise in what is good, and simple concerning evil.
20 And the God of peace will crush Satan under your feet shortly. The grace of our Lord Jesus Christ be with you. Amen.
simple = without mixture — without evil mixed in — relating to those who cause division and offense in the previous verses
crush Satan — again in reference to those antagonistic to Paul’s message
Verse 20 looks back to Genesis 3:15, the first prophecy. (And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise His heel.)
In the garden of Eden Satan came offering the knowledge of good and evil (Genesis 3:5). Here the believers are told, “I would have you wise as to what is good and guileless as to what is evil;” and then — get the importance of that word — “then the God of peace will soon crush Satan under your feet.” The crushing has to do with our victory in the sphere of the knowledge of good and evil, the same sphere in which sin first came to Adam and Eve. — Barnhouse, page 167
peace — as opposed to those who cause strife
shortly = with dispatch — when it happens, it will be over quickly.