For starters, there is no record of a Rabbi Amnon living in Germany in any other medieval literature. His name is not a typical Ashkenaz name (it is Italian), and the gruesome punishment of dismemberment was also not used in Germany in that time period.
Secondly, fragments of the UT have shown up in the Cairo Geniza to material that dates to the late 700s. That is some 300 years earlier than the martyrdom story purportedly took place!
Third, the piyyut contains a variety of notable similarities to a Christian poem called the “Hymn of Romanus upon Christ’s Reappearance,” which was written by a Jew who converted to Christianity in either the 500 or 700s. It shares a number of themes, motifs, and word patterns with this Christian hymn including trembling angels, God taking account of the souls of people, the blasts of the horn, books being opened, etc.
Fourth, the UT contains a variety of characteristics that can be identified with an earlier time period. The UT is a piyyut, and a piyyut is a very particular literary genre typified as a highly stylized poetic prayer. Piyyutim were not the standardized pieces of liturgy found in the classic rabbinic berakha framework; instead, they were designed to be additions inserted into berachot to enrich them.
The bit about how our poem seems to have been borrowed from a Catholic devotion is most interesting. Here, via the Shechter Institute, are some of the alleged parallels
|the angels shudder, fear and trembling sieze them||everything trembles|
|you open the book of records;|
you call to mind all things long forgotten
|the books are opened, the hidden things are made public|
|the angels shudder,|
they say it is the day of judgment
|the angels are dragged before the throne|
they cry: glory to Thee, most just judge!
|the great trumpet is sounded||upon the sound of the trumpet|
|they are not pure before thee||nobody is pure before thee|
|as the shepherd musters his flock, so do You|
cause to pass, number every living soul
|like a shepherd he will save|
|but repentance, prayer and tzedakah avert the severe decree.||Therefore, penitence and prayer will save you.|
The Catholic poem is from the 7th century (probably) and was written by a Jewish convert to Catholicism, raising the appealing possibility that Unetaneh Tokef dates to that period and was brought by the apostate into Christendom.
Another fun fact to know and tell: Unetanek tokef quotes Rosh Hashana 1:2 when it says all of man kind pass before God like כבני מרון, a phrase usually ranslated as "members of the flock." I've seen it suggested in many places that the word is actually kivinumeron, a Greek loan word meaning cohort of soldiers.
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