The minim questioned R. Simlay. They said to him: How many deities created the world? He said to them: Let us, me and you, inquire of the Torah. They said to him: See, it written (in Josh. 24:19): FOR … HOLY GODS.
He said to them: Read the rest of the verse, where "they" is not used but HE. R. Berekhyah said in the name of R. Abba the Edomite: Why is HOLY plural (in Josh. 24:19)? Because he is holy in all categories of holiness. R. Aha bar Hanina said: In regard to the Holy One, his way is in holiness; he processes in holiness; he is seen in holiness; his speech is in holiness; the uncovering of his arm is in holiness; he is beautiful and glorious in holiness.
Hence the plurals (in Josh. 24:19): FOR HE IS HOLY GODS, in the sense that he is holy in all types of holinesses. The minim said to him: But he caused to be written (in Gen. 1:1): IN THE BEGINNING GOD (in the plural) CREATED. He said to them: The plural form of "created" is not written here, but CREATED in the singular to agree with the singular subject.