The New Testament book of Hebrews contains an interesting metaphor: milk vs. solid food as "nutrients" (my word) for one's religious faith. The whole passage is 5:11-14.
About this we have much to say that is hard to explain, since you have become dull in understanding. For though by this time you ought to be teachers, you need someone to teach you again the basic elements of the oracles of God. You need milk, not solid food; for everyone who lives on milk, being still an infant, is unskilled in the word of righteousness. But solid food is for the mature, for those whose faculties have been trained by practice to distinguish good from evil.
I hate being talked down to and scolded, so I might have shamefully thought (if not said) "Screw you!" if someone told me this. But the passage is interesting and difficult because it follows some very sophisticated and nuanced theology in the previous 4-1/2 chapters!
We don't know the author of Hebrews nor the location of the congregation, although the latter were probably Jewish converts to Christianity (hence the title, added by later tradition), perhaps in Italy in the 60s AD. The congregation could've grasped and followed the writer's several arguments concerning the primacy of Christ, based on Old Testament scriptures (the only scriptures they had, of course). Yet, the author expresses concern that the people are still "babies." Interestingly, in 6:1-3, the author tells them to "leave [behind] the elementary teachings about Christ... not laying again the foundation of repentance... instruction about baptism," etc. (NIV). They know all that stuff! And yet the basic knowledge, as well as the more sophisticated teachings of the author (which you could consider "solid food"), are not leading the members to maturity (NIV: the NRSV has "perfection").
The author of Hebrews does alternate harsh warnings, reassurances and encouragement throughout the letter: for instance, the section coming up, 6:4-12. Why weren't the people mature? Their problem seemed to be apathy or sluggishness (6:12: cf. the "dullness" referred to in 5:11 above), weakness that implies weariness (12:12-13), and a tendency to "drift" (as an unanchored boat would drift: 2:1). They also seem to be facing a certain amount of persecution, though apparently not yet life-threatening (12:4). So the epistle's author felt the need to startle them: to talk louder, if you will, to get their attention.
This is interesting to me, partly because I want to be mature in Christ, too, and partly because this biblical advice is different from the way some of us preachers and churchgoers approach this subject. A denominational official visited our church several years ago and, I swear, he preached an evangelistic, "come to Jesus" sermon--for an established congregation. We pastors want people to grow in their faith but I think some of us try to do so by reiterating and reenforcing the "elementary teachings"---the "milk." Perhaps we should, instead, remind them of what they know (or should know) and push them toward deeper and more confident understanding.
What might jolt and lead people toward maturity? Discussing this Hebrews passage, the commentator in The New Interpreter's Bible (vol. 11, p. 72) notes that the problem with the congregation was essentially a social failure! "The [original] readers [of the epistle] have apparently pulled back from bold witness to outsiders and from exhorting and encouraging one another. The loss of a congregational conversation means a loss of hearing. Through lack of use faculties grow dull and the members regress to a former condition of immaturity."
But their failure was also a failure to sharing the blessings of Christ's own life. We can follow Christian teachings and have correct beliefs and yet fall short of a full relationship with the living person and living presence of Christ himself---no historically distant teacher known only through a book. Thus the Hebrews author gives his readers so many glorious passages about the sufficiency of Christ for people's needs, about Christ's tender, real and present care for struggling sinful people. The failure (but not an irreversible one) of the congregation is a two-sided coin: an apathy toward the living Christ and an apathy toward mutual encouragement and social involvement and witness.
All this dovetails with one of my favorite Bible passages, Ephesians 4:11-16.
The gifts he gave were that some would be apostles, some prophets, some evangelists, some pastors and teachers, to equip the saints for the work of ministry, for building up the body of Christ, until all of us come to the unity of the faith and of the knowledge of the Son of God, to maturity, to the measure of the full stature of Christ. We must no longer be children, tossed to and fro and blown about by every wind of doctrine, by people’s trickery, by their craftiness in deceitful scheming. But speaking the truth in love, we must grow up in every way into him who is the head, into Christ, from whom the whole body, joined and knitted together by every ligament with which it is equipped, as each part is working properly, promotes the body’s growth in building itself up in love.
Paul (or, some scholars argue, the author writing in Paul's name) similarly links Christian maturity to mutual support and encouragement and Christ's living power. Christian maturity can't take place apart from a loving (a genuinely loving) fellowship wherein people can build one another up.
Similarly, 1 Corinthians 3:1-9:
And so, brothers and sisters, I could not speak to you as spiritual people, but rather as people of the flesh, as infants in Christ. I fed you with milk, not solid food, for you were not ready for solid food. Even now you are still not ready, for you are still of the flesh. For as long as there is jealousy and quarrelling among you, are you not of the flesh, and behaving according to human inclinations? For when one says, ‘I belong to Paul’, and another, ‘I belong to Apollos’, are you not merely human?
What then is Apollos? What is Paul? Servants through whom you came to believe, as the Lord assigned to each. I planted, Apollos watered, but God gave the growth. So neither the one who plants nor the one who waters is anything, but only God who gives the growth. The one who plants and the one who waters have a common purpose, and each will receive wages according to the labor of each. For we are God’s servants, working together; you are God’s field, God’s building.
In the previous chapter, Paul depicts Christian maturity in a lovely way, and then in this section, he lowers the boom on the Corinthians, who think they're mature (and possibly would've thought Paul was describing them in chapter 2), but they actually are "infants"! Their "infancy" is, once again, a basic social failure combined with a failure to appreciate the living, working presence of Christ among them. Instead of being pleased at God's grace, the Corinthians were jealous, quarrelsome, prone to divine themselves among factions, and to glorify the work of particular people (in this case, Apollos and Paul) in a possessive, self-important way.
I never liked the expression, "There is no limit to what can be accomplished if it doesn't matter who gets the credit," which has been attributed to both Emerson and Harry S. Truman, among others. Half-humorously, I think the saying could be used by people to take all the credit and not acknowledge and thank the efforts of others! But if we apply the saying to Paul, we could paraphrase: God can accomplish amazing things as we avoid playing favorites with one another, dividing ourselves into factions, and work humbly together for one another's benefit! But those things imply a deep trust and mutual affection among believers in a congregation---goals that a church might set prior to other, more programmatic goals.
Back to Hebrews: we grow and become mature believers in Christ in so far as we encourage and support one another---every day, in fact (Heb. 3:13, 10:23-24). I don't seek such encouragement every day, and would feel needy if I did. Biblically, though, I should be seeking and giving daily encouragement for my faith (including periodic course-corrections)!
So should we all--but, as I say, it would require a high level of love, trust, and sincere concern within our circles of fellowship.