The Defining Mitzvos of Judaism
Relevant to why megillas Rus is read on Shavuos…
Yevamos 47a explains what we must teach a prospective convert:
ומודיעין אותו מקצת מצות קלות ומקצת מצות חמורות, ומודיעין אותו עון לקט שכחה ופאה ומעשר עני. ומודיעין אותו ענשן של מצות, … וכשם שמודיעין אותו ענשן של מצות, כך מודיעין אותו מתן שכרן… לא רוב טובה ולא רוב פורענות. ואין מרבין עליו, ואין מדקדקין עליו.
And we inform him about some minor mitzvos and some stringent ones, we inform him about the sins of [neglecting] leqet ((leqet: small amounts dropped during harvesting that must be left for the poor)) , shikhechah ((shikhechah: similar to leqet, but the sheaves or fruit that were “forgotten” and not picked at all)) , pei’ah ((pei’ah: a “corner” or end of the field which must be left for the needy to harvest.)) and maaser ani — the tithe to the poor ((Maaser ani is taken after terumah for the kohein and maaser for the leviim in the third and sixth years of the shemitah cycle.)) [all means by which one’s crop is shared with the poor as part of making one’s livelihood].
And we tell him the punishments of [violating] mitzvos… And just as we tell him the punishments of mitzvos, we tell them the giving of the reward… Not overly about the good, nor overly about the suffering. And we do not overwhelm him, and we do not become unduly meticulous with him.
(See also Rambam, Issurei Bi’ah 14:2, who prepends teaching them the basics of the faith.)
Notice that there are four mitzvos a candidate for conversion must know. Yes, we give them the big picture, but these specific mitzvos must be included. Without knowing that one must leave leqet, shikhechah and pei’ah and actively give maaser ani to the poor, one doesn’t “get” the basic picture of what Judaism is.
Much like the common idiom used to be that an Orthodox Jew kept “Shabbos, kashrus and taharas hamishpachah“, to Chazal, the basics of Judaism are leqet, shikhekhah, pei’ah, and ma’aser sheini.