When put together with II Corinthians 2:11, Paul is saying that a godly sorrow unto repentance can actually give Satan the opportunity to turn a person's feelings about his sin into an abnormal self-pity, which will destroy that despairing person's relationship with the church and with God. He can turn such a person into a bitter cynic. The Devil is that clever.
It does not end there. In addition, he can turn the righteous indignation of those who are offended by another's sin into bitter self-righteousness if they do not forgive and forget and move on. He gets people going and coming unless they are aware that he can turn something good into a ploy to destroy a person's relationship with God and the church.
These are not the only weapons that Satan has in his arsenal. Remember, we are involved in a war, and a general will employ every kind of ploy, device, tool, or contrivance to rout the enemy. He will use decoys, infiltration, subversion, propaganda, rumors, misleading leaks of information, and sometimes a frontal attack with diversions on the flanks.
Satan is no different. However, God makes sure to warn us of his subtlety. The Devil creates distractions and illusions to deflect us from reaching our goal. He has the ability to make things that are in God's purpose unimportant (for instance, material things or vanity) seem important, while eternal, spiritual things he makes seem unimportant, unnecessary, and unrealistic.
Knowledge of what he is like would be unnecessary if he could not affect us after baptism. Despite his earlier defeat at the hand of God as well as his defeat by our David, Jesus Christ, he is still seeking to destroy God. Even when he fails at that, he still wants to destroy God's purpose of having us inherit His Kingdom.