John Calvin Commentary Deuteronomy 27:9

"And Moses and the priests the Levites spake unto all Israel, saying, Keep silence, and hearken, O Israel: this day thou art become the people of Jehovah thy God." — Deuteronomy 27:9 (ASV)

The ground of his exhortation is again taken from the special favor of adoption, with which the God of Abraham had honored them. For there was nothing which should have more effectually stimulated them to obedience than that more-than-paternal love and the gratuitous kindness with which He had shown them favor beforehand. Although, at the same time, they were admonished in these words about the purpose for which they were separated from other nations.

For the conclusion he draws is that because they were received by God as His people, they, therefore, were under an obligation to keep His statutes. As Paul more plainly teaches, we are redeemed from all iniquity so that Christ might purify for Himself a peculiar people, zealous of good works (Titus 2:14).

Moreover, since the priests were now appointed, from whose lips the doctrine of the law was to be sought, they now come forward in God’s own name. They engage the people not only to respond to His generous calls upon them but also to obey His ministers.