John Calvin Commentary Hebrews 5:13

"For every one that partaketh of milk is without experience of the word of righteousness; for he is a babe." — Hebrews 5:13 (ASV)

For every one who uses milk, or, who partakes of milk, etc. He means those who from tenderness or weakness still refuse solid doctrine; for otherwise, one who is grown up is not averse to milk. But he here reproves an infancy in understanding, such as constrains God even to use the simplest, most childlike language with us.

He then says that babes are not fit to receive the word of righteousness, understanding by righteousness the perfection of which he will shortly speak. For the Apostle does not here, as I think, refer to the question of how we are justified before God, but takes the word in a simpler sense, denoting that completeness of knowledge which leads to perfection, a role Paul ascribes to the Gospel in his epistle to the Colossians (Colossians 1:28). As if he had said that those who indulge themselves in their ignorance preclude themselves from a real knowledge of Christ, and that the doctrine of the Gospel is unfruitful in them because they never reach the goal, nor even come near it.