God said to Noah, “This is the sign of the covenant that I have established between me and all flesh that is on the earth.”
Articles of agreement among men are usually sealed, that the covenants may be the more solemn, and the performances of the covenants the more sure, to mutual satisfaction. God therefore, being willing more abundantly to show to the heirs of promise the immutability of his councils, has confirmed his covenant by a seal (Heb 6:17), which makes the foundations we build on stand sure, Ti2 2:19.
Commenting on Genesis 9:12-17
And God said to Noah, this is the token of the covenant,.... Which is repeated for the greater confirmation and certainty of it, since the fears of men would be apt to run very high, especially while the flood was fresh in memory: which I have established between me and all flesh that is upon the earth: see Gen 9:9, it is highly probable, that...
And God said unto Noah, This [is] the token of the covenant, which I have established between me and all flesh that [is] upon the earth. (m) God repeats this often to confirm Noah's faith even more.