What does 1 Corinthians 3:1 mean?
This builds on two things Paul said in the previous chapter. First, he wrote that he and others taught God's wisdom among the mature, apparently referring to those who have come to God by faith in Christ and are ready for the deeper truths of God (1 Corinthians 2:6). He also described spiritual people as Christians who understand and believe in spiritual things with the help of God's Holy Spirit (1 Corinthians 2:14).Now, though, Paul writes to the Christians in Corinth that he cannot call them "spiritual" people. Is the problem that they have not truly believed in Jesus or that they have not received the Holy Spirit? No. Paul very clearly wrote in the first chapter how thankful he was that that their faith had been confirmed by the gifts of the Spirit given to them (1 Corinthians 1:4–9). He said they would absolutely stand blameless before God on the day of the Lord. These are Christians.
The problem, as Paul will go on to describe it in this chapter, is that they are still living as if they were unspiritual people: "merely human," (1 Corinthians 3:3), suggesting someone lacking understanding as if they lacked the Holy Spirit (1 Corinthians 2:14). They are like "infants in Christ"—spiritually speaking, they are still newborn, weak, undeveloped Christians. They continue to live in the flesh, meaning that they are living for self and their bodily appetites instead of living in the power God has given to them in the Holy Spirit.