The warning is against the borrowed mind. Systems handed down by other men, the inherited opinions and the first crude elements a person absorbs from the world, can quietly empty out the inner treasure and leave the reasoning faculty running on secondhand fuel. The rudiments are the ABC's of outer learning, the basic building blocks the world teaches, useful for the surface but powerless to reach the anointed center within.
Christ here is not a man far off in history but the risen faculty inside, the awakened higher mind that knows directly and does not need to be told. When thought is built on tradition and empty cleverness it stands on the outside of a person; when it is built on that inner anointing it stands at the true source. The difference is whether a person lives from what was poured into the head or from what wakes at the center.
Rudiments of the world: the crude first elements of outer learning, the surface knowledge that cannot reach the inner center
Christ: the awakened higher mind within a person, the anointed faculty that knows directly without being taught