- Is there a source for not verbally (or in writing, for that matter) mentioning the name of Yeshu (the father of Christianity)?
- If there is such a source, what exactly is forbidden to say? Is it only the English and/or the "full" pronunciation in hebrew (i.e. with an "a" after Yeshu - please verify the validity behind this as well) and not the hebrew (as i have heard that yeshu is just an acronym for yemach shemo vezichro), or is it even the name Yeshu itself? Please include sources, preferably with links
According to the sources cited by the Gra on Shulchan Aruch, Yoreh De'ah 147:3, the prohibition against mentioning the name of a foreign deity does not apply to the name of Jesus, and in fact we find that he is mentioned by name in many sources.
In a very interesting teshuva, R' Esriel Hildesheimer discusses this issue at some length. He comments that the prohibition against mentioning a foreign god's name applies only to speech, not to writing and that the prohibition does not apply to speech for the purpose of Torah study or psak halacha.
Nevertheless, R' Hildesheimer writes that the traditional practice has been to avoid explicitly mentioning the name of Jesus (referring to him instead as oso ha'ish - "that person"), and it is proper to uphold that practice.
R Meir hakohen (commonly known as the hagahot maimoniyot. rambam AZ chapter 5), clearly permits mentioning his name (The Rama cites him as well YD siman 147).
כתב רא"ם שאין אסור אלא שם שניתן לה לשום אלהות שמשמע אלהות, אבל שם הדיוטות כגון שמות בעלמא כשמות הגוים אע"פ שעשאהו אלוה כיון שבזה השם אין בו אלהות ואדנות וגם לא ניתן לו לשם כך מותר. וטעמא, דכתיב ושם אלהים אחרים לא תזכירו, בשם אלהות הקפיד הכתוב. וכן תנן אלו הן אידיהן של גוים קלנדא וכו' שאלו שמות הדיוטות הם, ובכמה מקומות בתלמוד הוזכרו ישו הנוצרי ותלמידיו, ואין אלוה גוים יותר ממנו
In short, if a name of a deity is used regularly and not in the context of divinity one is permitted to invoke it. The prohibition is only when he invokes the name in the context of divinity and when it connotes a deity, but if it is just a name given to a person or object the fact that its origin is pagan is insignificant and its use is permitted. For this reason he says the sages called Jesus by his name, since it was not used in such a context.