Adam Clarke on Hebrews 8:8

For finding fault with them - The meaning is evidently this: God, in order to show that the first covenant was inefficient, saith to them, the Israelites, Behold, the days come when I will make a new covenant, etc. He found fault with the covenant, and addressed the people concerning his purpose of giving another covenant, that should be such as the necessities of mankind required. As this place refers to Jer 31:31-34, the words finding fault with them may refer to the Jewish people, of whom the Lord complains that they had broken his covenant though he was a husband to them. See below.

With the house of Israel and with the house of Judah - That is, with all the descendants of the twelve sons of Jacob. This is thought to be a promise of the conversion of all the Jews to Christianity; both of the lost tribes, and of those who are known to exist in Asiatic and European countries.