For the lips of a priest guard knowledge, And people seek rulings from their mouth; For they are a messenger of God.
But Rabbi Meir- how did he learn Torah from the mouth of Acher ["the Other"]? But didn't Rabba the son of the son of Chana say, "Rabbi Yochanan said, 'What is [the meaning of] the verse, 'For the lips of a priest guard knowledge, And people seek rulings from their mouth; For they are a messenger of God'? If the teacher is like an angel of God, they should ask instruction from his mouth; And if not, they should not ask instruction from his mouth.'" ... There is no contradiction. This one (ie. when there is no need for a teacher like an angel) is with a great one, and this one (ie. when there is a need for a teacher like an angel) is with a lesser one.
When Rav Dimi came [from Israel], he said, "They say in the west (ie. Israel): 'Rabbi Meir ate a date and threw away the kernel.'" Rava expounded: "What is [the meaning of] the verse, 'To the garden of almonds I went down, to see the streams of water... (Song of Songs 6:11)-' why are scholars compared to almonds? To say to you, Just like an almond, even though it is dirty with mud and manure, what is inside it is not spoiled, so too a scholar, even though he has rotted, his Torah is not spoiled..."
(כב) המוסרים והאפיקורסים והמומרים פחותים מהעכו"ם ופסולים לעדות: הגה ואפי' מחל לו הנמסר פסול עד שישוב בתשובה (ב"י בשם מ"כ ובדברי הרמב"ם בפי"ב וכן משמע באשירי פ' זה בורר) מומר שחזר בו וקיבל עליו תשובה כשר מיד אע"פ שלא עשאה עדיין (מהרי"ק שורש פ"ה):
Snitches, heretics and mumarim are of a lower level than gentiles and are disqualified from testifying. A snitch cannot testify, even if the victim forgave him, until he repents. An apostate who agrees to repent is valid immediately even if he has not yet done it.
May 30, 1959.
Rav Moshe Feinstein zt”l pens a response to a Rebbe who is teaching in Rav Binyamin Kamenetsky’s Yeshiva of South Shore (IM EH Vol. I #96). The Rebbe, Rav Shmuel Dishon, had asked Rav Moshe about a certain individual artist who once had an excellent reputation. This artist had composed a number of musical compositions that had captured the hearts of the Torah community. Unfortunately, the artist had gone astray. Is it permitted to listen and sing the tunes that he had composed while he was still “fully kosher?”
Rav Moshe zt”l rules that there is nothing wrong with doing so for tunes that he had composed while he was “fully kosher.” For tunes that he had composed after his fall, Rav Feinstein writes, “it is likely that we should not be stringent since tunes do not essentially have to do with matters of Kedusha, however, Bnei Torah and Baalei Nefesh should avoid it.”