Hebrews 5:13

Immature babies consume only milk. They cannot chew and assimilate solid food because they are immature. Comparing milk and solid food was very common in Greek ethical philosophy. [Note: Moffatt, pp. 70-71.] Thus the readers would have had no question about the writer’s meaning. Similarly immature Christians take in only the basics of the gospel because they cannot receive and assimilate the more advanced aspects of the faith. They cannot do this because they have not tried repeatedly (practiced) to understand and apply these more advanced truths. This is a picture of Christians who have been content to know and practice only the most elementary lessons of their faith. They are too lazy to do what is necessary to grow. Of course, even mature adults continue to need milk, which is pre-digested food, but they can also eat solid food.