I have set my bow in the cloud, and it shall be a sign of the covenant between me and the earth.
13. I do set my bow in the cloud . From these words certain eminent theologians have been induced to deny, that there was any rainbow before the deluge: which is frivolous. For the words of Moses do not signify, that a bow was then formed which did not previously exist; but that a mark was engraven upon it, which should give a sign of the divine favor towards men.
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
I do set my bow in the cloud,.... Or "I have given", or "have set it" (p); which seems as if it was at that instant set; this is the same we call the "rainbow": and so Horace (q) calls it "arcus pluvius": it is called a "bow", because of its form, being a semicircle, and a "rainbow", because it is seen in a day...