John Gill Commentary Deuteronomy 7:9

"Know therefore that Jehovah thy God, he is God, the faithful God, who keepeth covenant and lovingkindness with them that love him and keep his commandments to a thousand generations," — Deuteronomy 7:9 (ASV)

Ver. 9 Know therefore that the Lord your God, he is God

The only true and living God, and not the idols of the Gentiles, who are false and lifeless ones, and therefore not the proper objects of adoration:

the faithful God, which keeps covenant and mercy ;
as appeared by fulfilling the promise made to their fathers, in bringing them out of Egypt, and now them to the borders of the land of Canaan given them for an inheritance:

with them that love him, and keep his commandments, to a thousand
generations ;
see (Exodus 20:6) which are not the causes or conditions of his covenant and mercy, nor of his keeping them, but descriptive of the persons that enjoy the benefit thereof.