Hebrews 5:12

For when by reason of the time ye ought to be teachers, ye have need again that some one teach you the rudiments of the first principles of the oracles of God; and have become such as have need of milk and not of solid food.

That the knowledge of spiritual things may be classified as elementary and advanced is explicit in the words "milk" and "solid food." Perhaps there is even a more advanced classification to be discerned in the words of Paul who said, "For the Spirit searcheth all things, yea, the deep things of God" (1 Corinthians 2:10). The metaphor of milk and solid food will be noted under Hebrews 5:13-14.

It should be noted specifically that there are certain elementary, basic, and foundational truths in the Christian system that should be known and received by all. These things are called here "rudiments" and "first principles"; and there is not the slightest hint in this place and elsewhere that these basic things are not important; but, on the other hand, they are projected as vital. Indeed, the author states that the people need to be taught all that again. "Ye have need again" that someone teach you, etc. An outline of Christian fundamentals is given in the next chapter, and each of them will be more fully studied there.