Hebrews 5:13

For everyone that partaketh of milk is without experience in the word of righteousness; for he is a babe. But solid food is for full grown men, even those who by reason of use have their senses exercised to discern good and evil.

Peter referred to young Christians as "newborn babes," admonishing them to "long for the spiritual milk which is without guile, that ye may grow thereby unto salvation" (1 Peter 2:2). Paul used the same metaphor and extended it thus, "I fed you with milk, not with meat; for ye were not able to bear it; nay, not even now are ye able; for ye are yet carnal" (1 Corinthians 3:2,3). The metaphor of children and full-grown men is also used in Ephesians 4:13,14, where Paul admonished his readers to attain the stature of "full-grown men" and that they should be no longer "children."

From the passage before us, it is plain that spiritual maturity is not simply a matter of time. Many who have been Christians many years may be in the condition of these Hebrew Christians. True spiritual growth is the result of prayer, study, meditation, faithfulness, diligence, exercise, and the successful struggle against temptations. The need for spiritual maturity is implicit in the confusion of the complex and sinful world in which people live. Matters of right and wrong do not always appear as checkerboard squares of black and white, there being many gray areas where the proper discovery of what is right and wrong can be a far more difficult matter. The great loss to the spiritual infant is that he may be misled, an eventuality that becomes certain unless he attains some degree of spiritual maturity. The church needs full-grown people, people who are not blown about by every wind of doctrine, people who have triumphed over the flesh, people of deep and loving personality, able to comfort the weak and the discouraged, and prepared to stand against all obstacles whatsoever. The pity of the present age of the church is seen in congregations of spiritual infants, uninstructed in the weightier things of the true faith, and indeed utterly ignorant of them, incapable of recognizing the most arrogant heresies, even those that deny the Lord, and still, after so many years, possessing only the most elementary knowledge of Christianity.