(Lev. 19:2:) “You shall be holy.” See what is written (in Josh. 24:19), “for He is a holy God (in the plural).” What is the meaning of this verse? It provides an opening for the heretics (minim), in that it seems to them like two powers. The heretics asked R. Simlay, “What is the meaning of ‘for He is a holy God?’ Do you not say that He is one power?
See from this verse, that there are [at least] two powers.” He said to them, “You idiots! Had it said, ‘[for] they are holy,’ you would have spoken [well. But] it is written, ‘[for] He (in the singular).’”
And [regarding] that which it says, “holy God (in the plural),” R. Berekhyah said in the name of R. Abba, “What is the meaning of ‘He is a holy [God] (with holy in the plural)?’ That He is holy in all categories of holiness.” How? R. Aha bar Hanina said, “His speech is in holiness, as stated (in Ps. 60:8), ‘God spoke in His holiness (i.e., in the holy place, the Temple).’
His way is in holiness, as stated (in Ps. 77:14), ‘Your way, O God, is in holiness (i.e., in the holy place).’ He is seen in holiness, as stated (in Ps. 63:3), ‘So I have beheld You in holiness, (i.e., in the holy place).’ His praise is in holiness, as stated (in Exod. 15:11), ‘Who is like You, glorious in holiness?’
The uncovering of His arm is in holiness, as stated (in Is. 52:10), ‘The Lord has uncovered His arm of holiness.’” Ergo (in Josh. 24:19) “He is a holy God (with holy in the plural),” because He is holy in all categories of holiness.