John Gill 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)

And Moses and the priests the Levites spoke to all Israel ,
&c.] The priests who were Levites, as all the priests that were lawful ones were. At this time, only Eleazar and Ithamar, and their sons, were such. These joined with Moses in the following exhortations to the people of Israel, as being particularly concerned in instructing them in the knowledge of the laws, and in seeing them put in execution:

saying, take heed, and hearken, O Israel ;
to what was about to be said to them, as well as to what had been said:

this day you are become the people of the Lord your God ;
they were his people before; he had chosen them to be his special people above all others; he had redeemed them out of Egypt; he had led them through the wilderness, and provided for them and protected them there, and had given them laws and statutes to observe to walk in; all which showed them to be his peculiar people: but now in a very formal and solemn manner they were avouched and declared by him to be his people, and they had solemnly avouched and declared that he was their God and King; and every day, according to Jarchi, was to be considered as this day, as if it was the day of entering into covenant with him.