Hebrews 5:11-14 (ESV)
About this we have much to say, and it is hard to explain, since you have become dull of hearing.  For though by this time you ought to be teachers, you need someone to teach you again the basic principles of the oracles of God. You need milk, not solid food,  for everyone who lives on milk is unskilled in the word of righteousness, since he is a child.  But solid food is for the mature, for those who have their powers of discernment trained by constant practice to distinguish good from evil.
Become dull of hearing, I think that means lazy. It is a shame. The Christian faith can occupy a lifetime of learning. And it doesn’t ever have to get boring. But sometimes it does. That is dangerous. It shouldn’t ever be boring.
The Christian faith is simple. It is so simple that a child can understand the sum total of it before he has weaned himself of milk. It isn’t such that you can never be certain of what you know, because there is always more to learn. We can be certain of much when it comes to the Christian faith, especially the grace of God in the forgiveness of sins. But there will always be more that we can learn, we can delve ever deeper into the basic principles of the oracles of God. Doing this we are trained by constant practice to distinguish good from evil.