In the Corinthians’ case, their spiritual immaturity is especially reflected in their partisanship. Instead of focusing on Christ, they focus on human leaders (see also 1 Corinthians 1:12). Paul cannot help but wonder if, after all, the Corinthians might be merely human.
In other words, he questions if they really have the Spirit. What a distressing thought!
4 For when one says, “I follow Paul,” and another, “I follow Apollos,” are you not being merely human?