About this we have much to say, and it is hard to explain, since you have become dull of hearing.  For though by this time you ought to be teachers, you need someone to teach you again the basic principles of the oracles of God. You need milk, not solid food,  for everyone who lives on milk is unskilled in the word of righteousness, since he is a child.  But solid food is for the mature, for those who have their powers of discernment trained by constant practice to distinguish good from evil.
(Hebrews 5:11-14 ESV)
John Piper comments:
Now this is amazing. Don’t miss it. It could save you years of wasted living. What verse 14 is saying is that if you want to become mature and understand the more solid teachings of the Word, then the rich, nutritional precious milk of God’s gospel promises must transform your moral senses—your spiritual mind—so that you can discern between good and evil. Or let me put it another way. Getting ready to feast on all God’s Word is not first an intellectual challenge; it is first a moral challenge…
The startling truth is that, if you stumble over Melchizedek, it may be because you watch questionable TV programs. If you stumble over the doctrine of election, it may be because you still use some shady business practices. If you stumble over the God-centered word of Christ in the cross, it may be because you love money and spend too much and give too little. The pathway to spiritual maturity and solid biblical food is not first becoming an intelligent person, but becoming an obedient person. What you do with alcohol and sex and money and leisure and food and computer have more to do with you capacity for solid food than where you got to school and what books you read…(HT: Derek Brown)