Hebrews 5:11 & 12
"We have much to say about this, but it is hard to explain because you are slow to learn. In fact, though by this time you ought to be teachers, you need someone to teach you the elementary truths of God's word all over again. You need milk, not solid food!" NIV translation
Paul goes on to say that there is much more to teach about this but it is for the Christian that is maturing. He contrasts the word of God to food for our body and explains that they still require milk like a baby. He also says that by now they should be able to teach others but they are not. Although this is not as important today as it was at the time, we should all be maturing and able to tell others about our faith. During Paul's time, it was vital because people did not have the scriptures on paper, their phones, or anything else.