"Those who bring the multitudes to righteousness will be like the stars forever and ever" - Daniyel 12:3
The Zohar HaKadosh says in Parasha Terumah that if only people knew the great level of those that fulfill the mitzvah of Zikuy Harabim, helping the children of HaShem, Am Yisrael - the Jews, come back and grow closer to Him, then they would have chased the mitzvah of Zikuy Harabim like they chase life itself. The Holy Zohar says that there is a special department in heaven that does not let anyone in except those that fulfill the mitzvah of Zikuy Harabim.
"It is proper for you to know, my brother, regarding the reward of the believer, that even if he reaches the furthest extreme in the rectification of his soul in its devotion to G-d and came near to the prophets, in their good traits, praiseworthy conduct, zeal in the service of G-d and pure love of Him, it is not like the merit of one who leads people to the good path, and straightens the wicked to the service of G-d, because his merits multiply according to their merits, every day and at all times. If one rectifies only himself, he will have a small merit. But if one rectifies himself and many other people, he will have his merit multiplied according to the merits of all the people who he rectified to G-d."
- Rabbenu Bahya ben Yosef Ibn Paquda, Sefer Chovot HaLevavot, Shaar Ahavat HaShem, Perek 6
"If a person will excel in the purity of Yirat Shamayim - Fear of Heaven - then they will rise higher and higher in the World to Come until they reach the Personal Angels of HaShem. But even though another person may not reach this first person's level of righteousness and good deeds, they can still get to an even higher level in the World to Come than the first person by doing Kiruv (helping people become religious and grow closer to HaShem), as it's merits are above everything. Even higher than the Angels of HaKadosh Baruch Hu."
- Rav Ovadia Yosef ZT"L
"If you take the precious out of the vile, you shall be as My mouth" - Yirmiyahu 15:19 Rashi explains: if you bring a wicked man to repentance and make him a righteous man, then your prayers will have the power to annul bad decrees.
"Hillel used to say: be of the disciples of Aaron, loving peace and pursuing peace, loving mankind and drawing them close to the Torah"
- Pirkei Avot 1:12
"Anyone that returns one Jewish soul, it is as if he created him" - Masechet Avot