This sutta details the exchange between Upali, a Niganthan (Jain), and the Buddha.
Upali explained the Jain view that there exist three kinds of "rods": bodily, verbal, and mental, of which bodily actions are the most reprehensible. The translators commentary explains that the three types of activities are considered to be instruments by which an individual torments himself and others. By stating that bodily actions are the most reprehensible, the Jains are implying that mental formations are not the forerunner of the other two actions.
In contrast, the Buddha states there are three kinds of "actions": bodily, verbal and mental, of which mental actions are the most reprehensible. Here, the Buddha is implying that mind is the forerunner of everything.
By the use of similes, Upali was won over by the Buddha, and became a lay follower.
Now, section 18 of the sutta is interesting, and it explains how Upali was able to become a stream-enterer. Firstly, the Buddha gave Upali "progessive instructions", i.e about giving, virtue, the heavens, and the danger of sensual pleasures and the blessing of renunciation. This made Upali receptive to the next crucial step: establishing him in stream-entry. In other suttas, laymen were given food to eat, to satisfy their hunger. So it seems that the Buddha takes pains to ensure that someone is in a receptive state to receive crucial instructions.
Here's how the sutta words it: "When he [the Buddha] knew that the householder Upali's mind was ready, receptive ... he expounded to him the teaching special to the Buddhas: suffering, its origin, its cessation, and the path".
In my previous blogs, I talked about how some people regarded all religions as essentially equal and equivalent; and how that view is unwarranted. MN56.18 gives a specific point of reference to refute the view of equivalency.
Section 18 goes further: "while the householder Upali sat there, the spotless immaculate vision of the Dhamma arose in him: 'All that is subject to arising is subject to cessation'". In other words, in that moment, Upali attained the path of stream-entry. I have a few things to say about the distinction between path and fruition stages, but I will leave that for another occasion.