Every baby needs to learn how to develop into the mature adult we were born to be and the same is true for the newborn child of God. Every baby was born with what is needed to grow into an adult, but it must be developed. The baby is born with the muscles of an adult, but they're not developed yet. These require the proper nourishment and care, in order to become a full-grown adult.
Babies must also develop their soul (emotions, mind and will). Unless the child's family is one of born-again believers, the child's spirit isn't developed. Peter writes about the new creation child of God in 1Peter2:2 (Amp), saying, "Like newborn babies you should crave (thirst for, earnestly desire) the pure, unadulterated, Spiritual milk of the Word, that by it you may be nurtured and grow unto completed salvation."
What is "completed salvation?" It's walking in a growing knowledge of our new identity in Christ, which requires learning what that identity is. Eph.4:22-24 (Amp) says, "Strip yourselves of your former nature (put off and discard your old unrenewed self), which characterized your previous manner of life which is becomes corrupt through lusts and desires that spring from delusion; And be constantly renewed in the spirit of your mind, having a fresh mental and Spiritual attitude. And put on the new nature (the regenerate self) created in God's Image (God like) in true righteousness and holiness."
Babies need milk when they're first born in order to grow, but their bodies must have solid food in order to develop strength and maturity. The child's structure and body is complete, but cannot develop or function without the solid food. James writes about our growth in Christ in Heb.5:13-14,12,11 (Amp), saying, "For everyone who continues to feed on Spiritual milk is obviously inexperienced and unskilled in the doctrine of righteousness (of conformity to the Divine will in purpose, thought and action) for he is a mere infant; not able to talk yet. But solid food is for full grown men, for those whose senses and mental faculties are trained by practice to discriminate and distinguish between what is morally good and noble, and what is evil and contrary either to Divine or human law."
Verse12 says, "For even though by this time you ought to be teaching others, you actually need someone to teach you over again the very first principles of God's Word, you have come to need milk, not solid food." I gave you Verses 12 before Verse 11, because this tells us the results from staying in the milk and not the Word. Thus, it says, "Concerning this we have much to say which is hard to explain, since you have become dull in your Spiritual hearing, and sluggish (even slothful in achieving Spiritual insight)." We've become content to feed on milk and not grow into responsible adult Christians.
We seem to reach adolescence and believe we know all there is to know. If you've ever raised a teenage child, then you understand what I'm saying. We eventually discover that there's more to life than what we thought as teenagers. Life's responsibilities are more than we understand as teenagers. It's not that what they know is wrong, but it's not complete. We must renew our minds by God's Word and His Spirit, in order to think according to those instead of our carnal minds and develop into the new creation men and women we were born to be in Christ Jesus.
Paul writes to the new creation people in Rom.8:6-7 (Amp), saying, "Now the mind of the flesh (which is sense and reason without the Holy Spirit), is death; death that comprises all the miseries arising from sin, both here and hereafter; but the mind of the Holy Spirit is Life and soul peace both now and forever. That is because the mind of the flesh with it's carnal thoughts and purposes is hostile to God, for it does not submit itself to God's Laws; indeed it cannot." This is why we must renew out minds by the meat of God's Word, as well as the milk.
In Rom.8:9 (Amp) Paul tells us why we not only should, but must renew our minds by God's Word, saying, "But you are not living the life of the flesh, you are living the Life of the Spirit, if the Holy Spirit of God really dwells within you, directs and controls you." We develop the mind of Christ and transform us from carnal thinking to Spiritual thinking, by and through the Holy Spirit living in us. The Holy Spirit will renew our minds from continue seeing ourselves as we once were and reveal who we've been made to be as new creations, according to Rom.8:1-2 (Amp), which says, "Therefore there is now no condemnation, no adjudging guilty of wrong, for those who are in Christ Jesus, who live and walk not after the dictates of the flesh, but after the dictates of the Spirit. For the Law of the Spirit of Life which is in Christ Jesus (the Law of our new being), has freed me from the Law of Sin and of Death." The renewed mind dwells on who Jesus has made us to be, instead of the carnal person we once were. We are spirit and made in God's Image, Who is Spirit.