'Solid' in the Bible

I gave you milk to drink, not solid food, because you weren't ready for it. And you're still not ready!

However, God's solid foundation still stands. It has this inscription on it: "The Lord knows those who belong to him," and "Everyone who calls on the name of the Lord must turn away from evil."

In fact, though by now you should be teachers, you still need someone to teach you the basic truths of God's word. You have become people who need milk instead of solid food.

But solid food is for mature people, whose minds are trained by practice to distinguish good from evil.