Professing believers who stay on a milk diet are unskilled in the word of righteousness. They are hearers of the word but not doers. They lose what they do not use, and remain in a state of perpetual infancy.
They do not have a keen sense of discernment in spiritual matters and are “tossed to and fro and carried about with every wind of doctrine, by the trickery of men, in the cunning craftiness of deceitful plotting”(Eph:4:14).
Solid spiritual food is for the full-grown, for those who by reason of use have their senses exercised to discern both good and evil. By obeying the light they receive from God’s word, these people are able to form spiritual judgments and save themselves from moral and doctrinal dangers.
William MacDonald, Believer’s Bible Commentary, Thomas Nelson
Publishers, 1995, p 2172