We come to Mishnah 18 in the fifth chapter of Pirkei Avos (in some versions it is Mishnah 21), which opens with a general rule: "Kol hamezakeh es harabim - ein cheit ba al yado" - whoever brings merit to the many, no sin comes through his hand. Let us begin with the plain meaning: "bringing merit to the many" means that a person takes other people and helps them acquire more merits.
The three ways in which a person acquires merit:
Refining one's character - improving one's traits and developing oneself.
Performing mitzvos - actual, practical deeds.
Torah study - although Torah study is itself a mitzvah, it is a distinct matter in its own right.
These are the ways in which a person earns merit for the World to Come. It follows that anyone who causes these things to happen in someone else is called "one who brings merit to the many":
In character: one who delivers effective words of mussar, who encourages people to be better and inspires them toward that.
In mitzvos: one who helps, enables and encourages others to perform mitzvos, and even provides them with the means to do so.
In Torah: one who inspires, teaches or enables another person to learn Torah.
In such a case it says: "ein cheit ba al yado" - no sin will come to his hand. The idea here is close to the principle that "one mitzvah brings another mitzvah in its wake," but this is a special payment and a special reward: a person who has done the great deed of helping others succeed spiritually is protected by the Holy One, Blessed is He, from sin.
This does not mean that the person's choice is limited in any way. His free will remains completely intact, and the Holy One, Blessed is He, does not prevent him from sinning or hold him back. After all, what makes us who we are, the very purpose of our existence and our defining quality, is the ability to choose. Therefore, under any ordinary circumstances the Holy One, Blessed is He, does not interfere with it.
Rather, here, as a special merit, Divine providence arranges that the opportunity to sin will be limited: the person is protected from bad things coming his way, things that would have tested him and might have caught him in the trap of sin. That is the meaning of the wording "ein cheit ba al yado" - it does not say that the person does not sin or cannot sin, but that he will somehow be protected from sin.
"Vechol hamachti es harabim - ein maspikin beyado laasos teshuvah" - and whoever causes the many to sin is not given the opportunity to repent:
Just as on the side of merit there is the principle that "one mitzvah brings another mitzvah," so here there is the principle that "one sin brings another sin," and this is a kind of additional punishment. A person who has committed the terrible crime of leading people astray, misleading them and preventing them from performing a mitzvah, has caused sin to be recorded in other people's accounts. Those who caused sins, who promoted sin and prevented good things from happening, receive a special punishment: that the opportunity to repent will not present itself to them.
Here too the intent is not that they cannot repent or that they will not repent. A person can always repent, and it is exceedingly rare - in fact almost impossible - for a person's free will to be taken from him to the point that he cannot return. The Rambam in Hilchos Teshuvah, chapter six, writes the famous words about the Holy One, Blessed is He, taking away Pharaoh's freedom of choice. But that is a most exceptional case: a person who committed sins so great that he deserved to be punished for them, and just as if the Holy One, Blessed is He, had put him to death his opportunity to repent would have ended, so too He arranged that Pharaoh would be unable to repent, because He needed him to remain defiant. But that is an extraordinary exception, and with any ordinary person it does not happen: whoever wants to return, the way is always open before him.
The usual rule is "one who comes to purify himself is helped": whoever seeks to set out on the good path and purify himself receives assistance from Heaven, for the Holy One, Blessed is He, desires his repentance and helps him. But here, one who committed not an ordinary sin, for which the path of teshuvah is always open, but the severe sin of "causing the many to sin" - having led others to sin - the Holy One, Blessed is He, does not help him return.
"Moshe zachah v'zikah es harabim - zechus harabim taluy bo" - Moshe attained merit and brought merit to the many, therefore the merit of the many is attributed to him:
Moshe Rabbeinu had merits of his own, and he also caused the many to have merits, and therefore "the merit of the many depends on him" - it is credited to his account. This line is not coming to prove the previous statement: the previous statement dealt with the result of special providence and merit, that one who brings merit to the many is protected from having sin come through his hand. That is one point.
Here a further point is introduced: one who encourages, assists, prods and helps others to do the right thing receives even more, and his action is greater even than the action of the one who actually does the deed. In other words, beyond the fact that one who brings merit to the many is shielded from sin, when those other people do the right thing, he receives a share in the merit of what they did. This is not a zero sum game: the Holy One, Blessed is He, is not limited in the amount of merit He grants. The one who performs the deed receives his full merit, and in addition merit is credited to the one who caused, assisted, prodded and set the good thing in motion.
This is what the verse says: "Tzidkas Hashem asah" - first, Moshe Rabbeinu himself did the right thing; "u'mishpatav im Yisrael" - and in addition, he receives merit as it were for all of Israel who follow Hashem's statutes and laws, because he brought them merit by teaching them Torah. It emerges that Moshe Rabbeinu receives his own merit, and also the credit for everyone else's fulfillment of Hashem's laws and statutes.
"Yeravam chata v'hechti es harabim - cheit harabim taluy bo" - Yeravam sinned and caused the many to sin, therefore the sin of the many is attributed to him:
Yeravam, who rose up against King Rechavam the son of Shlomo, broke away and brought about the separation of the northern kingdom from the southern one, thereby splitting the Jewish people in two. In order to prevent the people from traveling down through Yerushalayim to pray in the Beis HaMikdash and bring offerings, he effectively blocked the road and set up two calf idols in the northern part of Eretz Yisrael, in Beis El and in Dan. By doing so he caused people not to go up to the Beis HaMikdash by way of Yerushalayim, and he brought them to a great sin. He is the classic example of a person who causes others to sin.
This is what the Mishnah says: "Yeravam chata" - he committed sins of his own; "v'hechti es Yisrael" - he caused others to sin; and therefore "cheit harabim taluy bo," for the failures and sins of the others are attributed to him and he bears that liability as well. As the verse puts it: "al chatos Yeravam asher chata va'asher hechti es Yisrael" - first his own sins, and in addition, in the extra word "va'asher," a second set of liabilities that he receives for the sins of Israel.
The message that emerges from the Mishnah:
The enormous value of helping others succeed spiritually, in the way of one who brings merit to the many. This is emphasized in many places, and especially in Chovos HaLevavos: a person does great things out of gratitude for what the Holy One, Blessed is He, has done for him, but the Holy One, Blessed is He, is always giving to the world out of kindness, and it is very hard to get ahead in that race. Every good thing a person does is almost like the repayment of a debt of gratitude for what was already given to him in advance. In order to multiply one's merits and to get ahead in that race, a person should bring merit to the many and assist others, since he receives the merit for their deeds as well.
It emerges that this is a kind of multiplier: one who brings ten people to do the right thing has it all credited to his account, and his reward is multiplied. Therefore every person should go out and find the way in which he can bring merit to the many: helping others grow spiritually, become better people, keep more mitzvos and learn more Torah - and certainly, Heaven forbid, not to do the opposite.