New answers tagged heresy
The sources are vast, ranging from the Tanakh itself, through Tannaitic and Amoraic eras, and into the Geonic era (since you specifically asked for sources pre-rishonim). The Shema (Devarim 6:4) is of course the flagship statement of Jewish monotheistic faith (see also Devarim 4:35 and others). See also Yeshayahu 44:24 and 45:12; Tehillim 135:6; Daniel ...
The shema? But seriously, see hagiga 15a. Acher's heresy was stating that metatron was a power equal to hashem. Upon seeing metatron sitting in heaven, he proclaimed "There are indeed two powers in Heaven."
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