Adam Clarke on Hebrews 5:13

For every one that useth milk - It is very likely that the apostle, by using this term, refers to the doctrines of the law, which were only the rudiments of religion, and were intended to lead us to Christ, that we might be justified by faith.

The word of righteousness - Αογος δικαιοσυνης· The doctrine of justification. I believe this to be the apostle's meaning. He that uses milk - rests in the ceremonies and observances of the law, is unskilful in the doctrine of justification; for this requires faith in the sacrificial death of the promised Messiah.