'Solid' in the Bible

I gave you milk to drink, not solid food, for you were not yet able [to eat it]. But now you are still not able,

However, the solid foundation of God stands firm, having this seal: "The Lord knows those who are his," and "Everyone who names the name of the Lord must abstain from unrighteousness."

For indeed, [although you] ought to be teachers {by this time}, you have need of someone to teach you again the beginning elements of the oracles of God, and {you have need of} milk, not solid food.

But solid food is for the mature, who because of practice have trained their faculties for the distinguishing of both good and evil.