Adherents of the “sudden enlightenment” argument claim that insight into the true nature of self and reality occurs in a single flash of understanding, and that once a person experiences this insight, he can never become deluded again. This has been an extremely influential position within Zen Buddhism, but it raises questions that are difficult to answer.
If a Zen master believed to be truly enlightened is found to have committed crimes or indulged in ethical lapses, the sudden enlightenment school has a hard time explaining this. Was he never actually enlightened at all, or was his seemingly sinister behavior a form of “crazy wisdom” or a type of teaching trick? Believers in sudden enlightenment are inclined to excuse any sort of behavior on the part of the master, because acknowledging his serious moral failings calls the entire doctrine into question. Unfortunately, some masters have clearly abused this situation, and in some cases are still renowned as Buddhist teachers despite sleeping with their own students, drunk driving and other behaviors that would not normally be considered marks of spiritual enlightenment.
The school of gradual enlightenment sees spirituality as a process, characterized by an interaction between moments of insight and years of hard, daily work on the self. From this perspective, it's easy to see how a person could have a powerful and valid spiritual insight, yet still be subject to unresolved character flaws. This perspective does not put the Zen practitioner in the position of having to defend the unethical behaviors of a spiritual teacher.