The apostle first reveals to the church that their spiritual immaturity and foolish behavior caused him to teach them only the rudimentary principles of Christianity
And, I, brethren, was not able to speak to you as to those who are spiritual, but as to those who are carnal - even as to babes in Christ. I gave you milk to drink, and not meat; for you were not yet able to receive spiritual meat; and neither are you able now (1Corinthians 3:1 - 2, HBFV throughout).
Paul then later states, "We (those who preach the gospel) are fools for the sake of Christ, but you are wise in Christ . . ." (1Corinthians 4:10).
Irony is the use of words to convey a meaning that is usually completely opposite to its literal meaning. The Apostle Paul, no doubt with a twinge of humor, labels himself a fool (which he clearly was not) and the church as wise (their carnal actions showing they were not) to point out they needed to reassess their standing before God.
Foolish Attitudes
Paul, roughly eight months after writing the Corinthians, feels compelled to address, yet again, the church's persistent difficulties and foolish superior attitude. He ironically writes, "You think you are so wise - yet you listen gladly to those fools (false teachers); you don't mind at all when they make you their slaves and take everything you have, and take advantage of you, and put on airs, and slap you in the face" (2Corinthians 11:19 - 20, TLB).
The false wisdom of the Corinthians led them to overlook, and even knowingly allow, a variety of sinful behaviors. False apostles (2Corinthians 11:13 - 15), whom the church failed to challenge, not only spread lies but also deceitfully defrauded the brethren of their money (not unlike the Pharisees with their use of Corban). The church also allowed the kind of sexual perversion among its members that even the pagans rejected (1Corinthians 5).
Paul ends his correction by humorously and ironically stating he was too "weak and timid" to allow such destructive behavior among fools!