The following is an excerpt from The MacArthur New Testament Commentary on 1 Corinthians 3 .

And I, brethren, could not speak to you as to spiritual men, but as to men of flesh, as to babes in Christ. I gave you milk to drink, not solid food; for you were not yet able to receive it. Indeed, even now you are not yet able, for you are still fleshly. (1 Corinthians 3:1–3)

The cause of division in the Corinthian church was more than an external, worldly influence. It was also internal, fleshly. The Corinthians had succumbed to the pressures of the world, but they were also succumbing to the pressures and enticements of their own flesh.

Before Paul chastises them for their immature sinfulness, he reminds them again that he is speaking to them as brethren, as fellow believers. That is a term of recognition and of love. It reminded his brothers in Christ that they were still saved, that their sinning, terrible and inexcusable as it was, did not forfeit their salvation. He did not try to diminish the seriousness of their sins, but he did try to diminish or prevent any discouragement that his rebuke might otherwise have caused. He stood with them as a brother, not over them as a judge.

But Paul could not speak to the Corinthian believers as spiritual men. They had come through the door of faith but had gone no farther. Most of them had received Jesus Christ years earlier but were acting as if they had just been born again. They were still babes in Christ.

The New Testament uses the word spiritual in a number of ways. In a neutral sense it simply means the realm of spiritual things, in contrast to the realm of the physical. When applied to men, however, it is used of their relationship to God in one of two ways: positionally or practically. Unbelievers are totally unspiritual in both senses. They possess neither a new spirit nor the Holy Spirit. Their position is natural and their practice is natural. Believers, on the other hand, are totally spiritual in the positional sense, because they have been given a new inner being that loves God and is indwelt by His Holy Spirit. But practically, believers can also be unspiritual.

In 2:14–15 Paul contrasts believers and unbelievers, and his use of “spiritual” in that context refers, therefore, to positional spirituality. The “natural man” (v. 14) is the unsaved; “he who is spiritual” (v. 15) is the saved. In the positional sense, there is no such thing as an unspiritual Christian or a partially spiritual Christian. In this sense every believer is equal. This spiritual is a synonym for possessing the life of God in the soul, or as in 2:16, having the mind of Christ.

A positionally spiritual person is one with a new heart, indwelt by and controlled by the Holy Spirit. “You are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. But if anyone does not have the Spirit of Christ, he does not belong to Him” (Rom. 8:9; cf. v. 14). When we trust in Jesus Christ, His Spirit takes charge of our lives and remains in charge until we die. He will control us to His own ultimate ends, whether we submit or not. “We know that God causes all things to work together for good to those who love God, to those who are called according to His purpose” (Rom. 8:28). Our resistance and disobedience can cause many unnecessary detours, delays, and heartaches, but He will accomplish His work in us. “he who began a good work in you will perfect it until the day of Christ Jesus” (Phil. 1:6).

Practically, however, believers may be anything but spiritual. Such were the Corinthian Christians. Paul addressed them as brethren. but he made it clear that he had to speak to them on the lowest possible spiritual level. He had to speak to them as if they were men of flesh.

Men of flesh (sarkinos) is literally “fleshy ones.” In this context it refers to man’s fallen humanness, his Adamic self—his bodily desires that manifest rebelliousness toward God, his glorying in himself, and his proneness to sin. As mentioned above, the flesh is not eradicated when we are saved. It no longer can ultimately dominate or destroy us, but it can still greatly influence us. That is why we yearn for the redemption of the body (Rom. 8:23). Glorification, in one sense, will be less of a change than justification. Justification was transformation of the inner being; glorification is the elimination of the outer being, which bears the curse.

So a Christian is not characterized by sin; it no longer represents his basic nature. But he is still able to sin, and his sin is just as sinful as the sin of an unbeliever. Sin is sin. When a Christian sins, he is being practically unspiritual, living on the same practical level as an unbeliever. Consequently Paul is compelled to speak to the Corinthian believers much as if they were unbelievers.