Like newborn infants, long of the pure spiritual milk, that by it you may grow up into salvation
1 Peter 2:2 ESV
Although my second child still less than a week old, I am already joyfully learning from her. You see, this verse keeps coming to my mind as I continuously observe her devoting herself essentially to one activity: feeding. Peter here calls us to imitate spiritually this element of the physical behavior of a newborn infant as they long for their mother’s milk. Indeed, longing is a very fitting description, since a newborn knows little other than their own hunger and the satisfaction of being filled. Apart from feeding, they spend most of their time sleeping, and it is that combination of nutrition-dense milk and rest that enables them to grow beyond their helpless state as newborns.
The apostle calls us to imitate their restless desire for their mother’s milk by longing desperately for the pure spiritual milk, which is almost certainly “the living and abiding word of God” that Peter referenced in 1:23. Just as an infant is utterly dependent upon milk for the nutrition to grow, all followers of Christ are dependent upon the Scriptures for growth into salvation. Donald Whitney rightly notes that:
No factor is more influential in making us more like the Son of God than the Spirit of God working through the Word of God. If you want to be changed, if you want to become more like Jesus Christ, discipline yourself to read the Bible.
Indeed, becoming more like Jesus is the only true growth of a Christian. In Ephesians 4:15, Paul declared that the church (both collectively and as individual believers) must set out to “grow up in every way into him who is the head, into Christ.” The Scriptures, therefore, are the milk that fosters our growth because they alone reveal Christ to us (John 5:39). In fact, just as a newborn will cry until satisfied with their mother’s milk, so too will a true believer fail to be satisfied by anything other than the Word of God.
If Christ is your supreme treasure and your life, then you must long to be more like Him. And if, as for John the Baptist, you long to see Christ increase as you decrease (John 3:30), then you will hunger to know Him more in His Word. Therefore, like a newborn, forsake any pride of self-sufficiency and immerse yourself with the Scriptures, crying for your Father to nourish you little by little into “mature manhood, to the measure of the stature of the fullness of Christ” (Ephesians 4:14).
 Donald Whitney, Spiritual Disciplines, 28.