Matthew Henry Commentary Deuteronomy 9:1-6

"Hear, O Israel: thou art to pass over the Jordan this day, to go in to dispossess nations greater and mightier than thyself, cities great and fortified up to heaven, a people great and tall, the sons of the Anakim, whom thou knowest, and of whom thou hast heard say, Who can stand before the sons of Anak? Know therefore this day, that Jehovah thy God is he who goeth over before thee as a devouring fire; he will destroy them, and he will bring them down before thee: so shalt thou drive them out, and make them to perish quickly, as Jehovah hath spoken unto thee. Speak not thou in thy heart, after that Jehovah thy God hath thrust them out from before thee, saying, For my righteousness Jehovah hath brought me in to possess this land; whereas for the wickedness of these nations Jehovah doth drive them out from before thee. Not for thy righteousness, or for the uprightness of thy heart, dost thou go in to possess their land; but for the wickedness of these nations Jehovah thy God doth drive them out from before thee, and that he may establish the word which Jehovah sware unto thy fathers, to Abraham, to Isaac, and to Jacob. 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 9:1-6 (ASV)

Moses represents the strength of the enemies they were now to encounter. This was intended to drive them to God and engage their hope in Him. He assures them of victory through the presence of God with them. He cautions them not to entertain the slightest thought that their own righteousness procured this favor from God.

In Christ we have both righteousness and strength; in Him we must glory, not in ourselves, nor in any sufficiency of our own. It is for the wickedness of these nations that God drives them out. All whom God rejects are rejected for their own wickedness; but none whom He accepts are accepted for their own righteousness. Thus, boasting is forever done away with (see Ephesians 2:9, 11, 12).