פּירוש אויף דברים 13:19

The verse for you will follow… means: So will you do, if you seek to practice what is right in His eyes. This passage is juxtaposed to the subsequent passage on account of the enticers to idolatry (be they brother, or son, or father, or the inhabitants of the “seduced city”) — it is forbidden to mourn over them at all, and certainly to cut one’s flesh, which is forbidden for any dead person.

(Ibid. 19) "when you hearken to the voice of the L-rd your G-d to keep…": From here they said: If a man begins to hear a little, in the end, he merits hearing much. If he begins to hear, in the end he keeps.

"to keep all of His mitzvoth that I command you this day": that a "slight" mitzvah be as beloved by you as a "weighty" one.