Albert Barnes Commentary Deuteronomy 26:16-19

"This day Jehovah thy God commandeth thee to do these statutes and ordinances: thou shalt therefore keep and do them with all thy heart, and with all thy soul. Thou hast avouched Jehovah this day to be thy God, and that thou wouldest walk in his ways, and keep his statutes, and his commandments, and his ordinances, and hearken unto his voice: and Jehovah hath avouched thee this day to be a people for his own possession, as he hath promised thee, and that thou shouldest keep all his commandments; and to make thee high above all nations that he hath made, in praise, and in name, and in honor; and that thou mayest be a holy people unto Jehovah thy God, as he hath spoken." — Deuteronomy 26:16-19 (ASV)

A brief and earnest exhortation by way of conclusion to the second and longest discourse of the book.

Deuteronomy 26:17: You have avouched - literally, “made to say”: so too in the next verse. The sense is: “You have given occasion to the Lord to say that He is your God,” i.e., by promising that He will be so. Compare Exodus 24:7; Joshua 24:14–25.