13 Anyone who lives on milk, being still an infant, is not acquainted with the teaching about righteousness.

Read Hebrews 5:13 Using Other Translations

For every one that useth milk is unskilful in the word of righteousness: for he is a babe.
for everyone who lives on milk is unskilled in the word of righteousness, since he is a child.
For someone who lives on milk is still an infant and doesn’t know how to do what is right.