After the completion of the Babylonian Talmud in the 6th century CE, the heads of the major rabbinic academies of Mesopotamia—scholars known as g’onim—became widely acknowledged as the religious leaders of world Jewry. It was during this period that the term hazan ceased to refer to a synagogue functionary and came to refer to a new kind of prayer leader, who distinguished himself by singing hymns. Tamar Marvin explains this transition:
A central factor was the continued decline of Hebrew knowledge among laypeople. . . . Earlier, the Shabbat experience at synagogues was defined by the delivery of formal sermons, some of which are preserved, many in altered form, in the classical homiletical Midrash collections. Now, congregants evidently began to prefer piyyut (liturgical poetry) in place of the sermon—not because it was more understandable but because it was pleasantly sung.
In concert with these developments, there was the push of late Roman policy, which under the Christian emperors Theodosius and Justinian limited or outright persecuted pagans, Jews, and other religious minorities. In particular, Justinian’s Novella 146, issued in 553, dictated that Jews were to use Greek as the language of [sermons], an attempt to undercut the interpretive translation of Hebrew, which congregations could largely no longer understand, as well as the Oral Tradition upon which the interpretations relied. The limitations put in place by Novella 146 may have spurred the development of piyyut with its unique ability to convey midrashic interpretations in sung form that [would not] contravene Justinian’s legislation.