Can a ba'al teshuvah, someone who grew up non-observant, become a pulpit rabbi?
In the old days, the rabbi's job was simply to answer halakhic questions and another functionary -- a maggid or a mokhi'ah -- would speak to the public, inspire them and lead them to greater levels of observance. Today, however, this is the rabbi's job. Is everyone qualified to do this?
The Rambam (Mishneh Torah, Hilkhos Teshuvah 4:2) writes:
It must be appointed in each and every Jewish community one who is wise, great, learned, God-fearing from his youth and who is loved by everyone to rebuke everybody and make them repent.Note the requirement that he be "God-fearing from his youth." This would seem to exclude a ba'al teshuvah from becoming a pulpit rabbi.
However, there is a difficulty with the Rambam's position. The midrashic tradition has it that the prophet Yoel (Joel) was the son of Shmuel (Samuel). According to 1 Samuel 8:1-3, Yoel sinned. Now one could suggest that since the Gemara (Shabbos 56a) states that Shmuel's sons did not sin, they were entirely free from any sin. On this, see R. Avigdor Nevenzahl's collection of essays on Genesis (last essay) and R. Ya'akov Medan's book David u-Bassheva. If Yoel did, in fact, commit some sort of sin, even one of lesser severity than that mentioned in the text, then how could he become a prophet who rebuked the people? After all, he was not "God-fearing from his youth"?
R. Yitzhak Sorotzkin addresses this in his Rinas Yitzhak to Joel 1:1. He suggests that perhaps prophets are different and do not need to be God-fearing from youth. However, his second suggestion is relevant to our original question. The Gemara (Yoma 86b) states that one who repents from fear has his intentional sins turned into accidental sins and one who repents from love has his intentional sins turned into merits. Thus, suggests R. Sorotzkin, Yoel repented from his misdeeds through love and, therefore, his past sins were erased and he was as if he had been God-fearing throughout his life.
Based on this, we can suggest that someone who become observant out of a love for Torah, rather than fear of divine punishment, has his past sins erased and, like Yoel, is considered as if he has been God-fearing throughout his life. Therefore, he is qualified to become a pulpit rabbi.
However, someone who remains an unrepentant sinner, such as a practicing homosexual, cannot serve as a pulpit rabbi.