Hebrews 5:11-13

of whom we have much to say, and 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 first principles of the oracles of God; and you have come to need milk and not solid food. For everyone who partakes only of milk is unskilled in the word of righteousness, for he is a babe.