[Angelino97]

“Do not add to the word which I command you, nor diminish from it, to observe the commandments of the Lord your God which I command you.”

Isaiah was written after Deuteronomy.

So how do you know that Isaiah isn't adding to the word of God?

(Of course, I think both Isaiah and the New Testament are the Word of God.)

[Tom Tetroxide]

The Writings and the Prophets are commentary on the Torah.