If for just the duration of a finger-snap a monk indulges in a thought of goodwill and cultivates a thought of goodwill, he is verily to be called a monk. Earnestness, above all other things, encourages the arising of good states; indulgence, worst of all things, encourages evil states (A.i.10).
It is said (AA.i.40) that this sutta was preached at Jetavana to counteract the effects of the Aggikkhandhopama Sutta, which frightened many monks, causing them to revert to the lay life, the Order being thereby greatly impoverished in numbers. The present sutta was designed to reassure those who were in doubt.