Mahaa Moggallaana is visited by a deva who tells him of Devadatta's secret ambition to rule the sangha. When he relates this to the Buddha, the Buddha discourses on the various sorts of teachers that need the protection of their disciples and he declares himself not to need such protections.
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Index to available translations: AN 5.100
This is a deep sutta.
First Mahaa Moggallaana is told to guard his words; that is to say to not reveal this episode further.
And what is the relationship between what Mahaa Moggallaana reveals and the dissertation on the various sorts of teachers who are guarded by their disciples?
What the Deva has revealed to Mahaa Moggallaana is likely the very first formation of Devadatta's deviation.
At such a point it is possible for a person to give up his corrupt idea.
Should his inner thinking become widely known the tendency would be to stick to it rather than give it up.
Thus the compassionate thing to do under these circumstances would be to keep quite and hope for the best.
The sutta is deeper than this.
The second half explains Gotama's warning in the first.
It is a masterpiece in the art of the extremely subtle message.
Such art is very handy in the case of a group of people whose skill set includes that of mind-reading. Such teachers as are incomplete have tempers. They may get angry at having their flaws exposed. They have benefactors and their disciples guard them. Those benefactors may be powerful and become instruments of revenge.
Alternatively, no matter the feebleness of the kamma, they are doing good by giving and they might stop.
Further, the disciples count on gifts from their teacher. There is the danger that a pre-mature revelation of a flaw could be seen as a false accusation.
And who is who to damage the reputation of another?
The true nature of a person will out through his behavior without need of any external prompting.
For some of the devices of the evil bhikkhu, see AN 5.103.
There will be those who find this construction impossible to accept. I welcome their understanding of this sutta.