Charles Ellicott Commentary Deuteronomy 9:6

"Know therefore, that Jehovah thy God giveth thee not this good land to possess it for thy righteousness; for thou art a stiffnecked people." — Deuteronomy 9:6 (ASV)

Understand therefore. —Literally, and you know. Three times this formula occurs in these verses: “The children of Anak you know; and you know the Lord your God; and (thirdly) you know yourself too.”

A stiffnecked people. —The metaphor seems to be taken from a camel or other beast of burden, which hardens its neck and will not bend it for the driver.