According to St. Paul’s inspired epistles, the human race is divided into four categories:
1⃣ The natural man: That is, the fallen son of Adam, as he is, without God. Of him the Apostle says: “The natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned” (I Cor. 2:14).
2⃣ The babe in Christ: When a person sees himself as a sinner and trusts Christ as his Saviour, he is “born again” and becomes a “babe in Christ”. But babes can and should grow, so these are exhorted: “As newborn babes, desire the sincere [pure] milk of the Word, that ye may grow thereby” (I Pet. 2:2).
3⃣ The carnal Christian: One who, though perhaps a Christian for years, has not grown, due to indifference and neglect of the Word of God. He still has to be treated as a babe in Christ. The Corinthian believers were examples of this. Paul had to write them: “I…could not speak unto you as unto spiritual, but as unto carnal, even as unto babes in Christ. I have fed you with milk, and not with meat: for hitherto ye were not able to bear [digest] it, neither yet now are ye able” (I Cor. 3:1,2). Everybody loves a baby, but the joy that fills the hearts of loving parents turns to bitterest sorrow if their baby fails to grow.
4⃣ The spiritual Christian: One who, through prayerful study of the Word of God, has grown to spiritual maturity. He is no longer merely a child of God; he is a “man of God”. We should all “desire the sincere [pure] milk of the Word that ye may grow thereby” (I Pet. 2:2) — “THAT WE HENCEFORTH BE NO MORE CHILDREN, TOSSED TO AND FRO, AND CARRIED ABOUT WITH EVERY WIND OF DOCTRINE” (Eph. 4:14). Let us then heed St. Peter’s inspired exhortation: “BUT GROW IN GRACE, AND IN THE KNOWLEDGE OF OUR LORD AND SAVIOUR, JESUS CHRIST” (II Pet. 3:18). source