Swami Kriyananda,
in The Essence of the Bhagavad Gita Explained:

Sincere aspirants often wonder, “If Kriya Yoga is such an exalted science, why has it not been published in book form, so that all might learn it?” A reasonable question, certainly. The masters themselves, however, have decreed it should not be published, precisely because it is a teaching that transcends reason itself. Right understanding of it depends on the unfolding intuition.

Kriya Yoga, in order to be wholly effective, must be received not only intellectually (in written or spoken form), but vibrationally, in the form of initiation. A magnet is created either by electrical realignment of the molecules, or by close proximity to another magnet. Attunement with a God-awakened guru influences the samskaras (comparable to the material molecules) to flow upward to the brain.

We are dealing here with a reality subtler, and much more difficult to master, than mere molecules of metal. Without an experienced guide, even mountain climbing can be fatal—though death, in this case, only ends a single incarnation. Spiritual mistakes can be costlier, in terms of long-range suffering.

Guidance from the guru is not only helpful: It is essential. This does not mean that Kriya Yoga is dangerous. Far from it. But to take up Kriya Yoga signifies entering seriously onto the path to God.