These are the strong words of Paul to the Church at Corinth (1 Cor 3.3). Paul expects something better, something different from those who claim to follow Christ. In Chapter 6, after listing some sinful behaviour, he comments, “that is what some of you were” (6.11). The verb is in the past tense. Such behaviour is past, not present. You used to do that, but through Christ you stopped.
Back in the book of Leviticus, the Lord said to Moses; “if anyone sins and is unfaithful to the Lord by deceiving his neighbour about something entrusted to him or left in his care or stolen, or if he cheats him, or if he finds lost property and, lies about it, or if he swears falsely, or if he commits any such sin that people may do …” (Lev 6.1-3). Put another way; just because others do that, does not mean it is ok. What is ‘normal’ behaviour is often sinful behaviour. Common usage does not make something acceptable.
Behaving like mere men is not the same as behaving like Christ-followers. The Lord Jesus set a high bar. To His followers, He said; “unless your righteousness surpasses that of the Pharisees and the teachers of the law, you will certainly not enter the kingdom of heaven” (Matthew 5.20). Whilst we are not to compare ourselves to one another, nor consider ourselves better than others, the command is to “be perfect” (Matthew 5.48).