Theory thoroughly known by heart does not equal having actual realization.

Any theory, once you have understood it, comprehended it, grasped it, come to know it, learned to apply it, can answer questions about it fluently, can explain it to others, can explain it clearly, and have it perfectly clear in your mind—

yet, does that amount to having realized it through actual practice? Does it amount to the theory and views truly being your own, amounting to your own insight?

Reaching the point where a theory is thoroughly familiar to you is not difficult; answering fluently and drawing inferences from one instance to others is also not difficult. What is difficult is being able to advance to the stage of verification and realize it step by step through actual practice; and what is most difficult is being able to pass smoothly through the stage of verification and truly attain realization through actual practice.