Answer: The Gemara (Chagiga 15a) recounts the story of Elisha ben Avuya, also nicknamed “Acher” (the other one), one of the teachers of Rabbi Meir Ba’al Ha’Ness. He was a tremendous genius in Torah who took a downward spiral in spirituality until he eventually became completely non-observant and even desecrated the holy Shabbat in public. However, due to his lofty stature as a result of the Torah knowledge he had amassed, he was able to hear a Heavenly voice proclaim: “Return, my wayward sons, besides for Acher.” This means that he clearly heard a Heavenly voice announce that Hashem invited all sinners to repent, as Hashem exclaims lovingly, “Return to me! Return to me and I shall return to you!” Nevertheless, the Heavenly voice did not invite the soul of Acher, for he was so wicked in the eyes of Hashem that the gates of repentance were closed to him.
The Gemara explains at length how Rabbi Meir, the student of Acher, and later Rabbi Yochanan exerted much effort in order to save the soul of Acher from the punishment that awaited him in Heaven. It would seem then that Elisha ben Avuya was truly in a situation where he could no longer repent, for the Heavenly voice declared that he should not do so because Hashem has no interest in such wicked people.
Nevertheless, this is an incorrect way to think. One of the greatest Acharonim, Rabbeinu Yosef Tarani zt”l, writes in one of his responses that Acher’s mistake was that even though he heard the Heavenly voice telling him not to repent, he should have nonetheless repented. Similarly, our Sages teach us that “whatever one’s host tells him to do, one should do so, besides for if the host asks him to leave.” This alludes to the fact that Hashem is our Host and whatever He tells man to do, one must listen, besides for if Hashem asks one “to leave,” i.e. not to repent, then one must not listen, for Hashem does not wish for a sinner to die; rather, Hashem wishes that he repent and live. If so, why did the Heavenly voice invite all wayward sons besides for Acher?
Maran Ha’Chida answers in the name of the great Rabbeinu Avraham Yitzchaki who would explain that the intention of the Heavenly voice was to encourage all sinners to repent besides for Acher since he was such a great man and he understood the importance of abiding by the Torah and Mitzvot and he nonetheless went and became corrupt. As a result, the claim against him was so much greater and so was Hashem’s anger which caused the Heavenly voice not to call on him to repent. On the other hand, were he to repent on his own, his repentance would certainly have been accepted and he would once again have been beloved by Hashem. As Rabbi Meir told him, “Nothing stands in the way of repentance.”
Similarly, the Mekubalim explain the saying that one who has transgressed very serious prohibitions is unable to repent is not to be taken literally, G-d forbid, and one cannot repent at all; rather, this means that such an individual’s repentance process is much more difficult and he must strengthen himself to subjugate his evil inclination to repent fully and whole-heartedly. Even if it seems that one cannot mend his ways regarding this particular sin, one must know that this is the persuasion of the evil inclination. One should, at the very least, minimize one’s transgressions of such prohibitions, for every small effort on one’s part is very precious to Hashem.
Based on the above, even if an individual has transgressed grave prohibitions and caused the public to sin as well about whom our Sages say that Hashem does not aid one in repenting, nevertheless, one must try with all his might to overcome one’s evil inclination and sinful tendencies in order to repent fully on his own. If one does so, one is guaranteed by Heaven that Hashem shall accept his repentance willingly and lovingly.