Hebrews 5:12 in the King James Version
For when for the time ye ought to be teachers, ye have need that one teach you again which be the first principles of the oracles of God; and are become such as have need of milk, and not of strong meat.
Context
11 About him we have many words to say, and hard to interpret, seeing you have become dull of hearing.
12 For although by this time you should be teachers, you again need to have someone teach you the rudiments of the first principles of the revelations of God. You have come to need milk, and not solid food.
13 For everyone who lives on milk is not experienced in the word of righteousness, for he is a baby.