'Milk' in the Bible

I fed you with milk and not with solid food, since for this you were not yet strong enough. And even now you are not strong enough:

What soldier ever serves at his own cost? Who plants a vineyard and yet does not eat any of the grapes? Or who tends a herd of cattle and yet does not taste their milk?

For although, considering the long time you have been believers, you ought now to be teachers of others, you really need some one to teach you over again the very rudiments of the truths of God, and you have come to require milk instead of solid food.

By people who live on milk I mean those who are imperfectly acquainted with the teaching concerning righteousness.

Thirst, like newly-born infants, for pure milk for the soul, that by it you may grow up to salvation;