--- FACT: the reason people fail to learn is for a lack of repetition
Outside of born geniuses, the rate at which we learn isn't most contingent on our ability to put stuff in, but rather, on our ability to keep it from falling out.
More precisely, there's a rift between short-term memory and understanding, which makes our natural forgetting curve very steep.
The basic idea is that an equal amount of memory work is more effective when spaced out than when concentrated; 60 mins at a go is less effective than 10 mins every day for 6 days.
What's more, there is an optimal duration per session, and an optimal spacing between such sessions, which varies according to the task.
The Catholic Church gets this right with religion. The timely reminder each Sunday makes their teachings a matter of doctrine within. As Dan Denett would say, every time you say it, you make another copy in your mind.
This is especially true when we're trying to start a movement and do something great in this world.
In any movement, there are it's key members, but most of the time, insight and vision lies with a single individual. And unlike the Bible, we don't have the luxury of 2,000 years to write it down.
Part of being a great leader is being able to inspire people towards your vision. And great leaders do that by constant reminders, both through actions and words.
And in real life, not all your followers can be on the same conceptual page. Hence, we need enough repetition to update as many people to your present vision without sounding like a broken record.
In other words, preaching to the choir is a valuable skill.