Hebrews 8:7

‘For if that first covenant had been faultless, then would no place have been sought for a second.’

And this replacement of the first covenant was clearly as necessary as was the changing of the priesthood (Hebrews 7:11), as is seen by the fact that Jeremiah in Scripture had declared the making of a new covenant (Jeremiah 31:31-34). For if such a new covenant was Scripturally required it openly demonstrated that the old covenant was lacking. Had it not been so, no new covenant would have been required. Thus the writer now quotes the new covenant in detail, mainly but not fully as per LXX. He may well have been paraphrasing LXX from memory. It also closely follows the Hebrew text.