And God said,
Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.
All Commentaries on Genesis 1:6 Go To Genesis 1
A firmament: Strabus and Bede teach that there is an eternal heaven, because the firmament, which they take to mean the sidereal heaven, is said to have been made, not in the beginning, but on the second day: whereas the reason given by Basil is that otherwise God would seem to have made darkness His first work. Augustine says (Gen. ad lit. i, 9) that the heaven of the second day is the corporeal heaven. According to Damascene (De Fide Orth. ii) the firmament made on the second day is the starry heaven. Chrysostom understood that the heaven in 1:1 is the same heaven of the second day.
Divide the waters from the waters: Whether, then, we understand by the firmament the starry heaven, or the cloudy region of the air, it is true to say that it divides the waters from the waters, according as we take water to denote formless matter, or any kind of transparent body, as fittingly designated under the name of waters. For the starry heaven divides the lower transparent bodies from the higher, and the cloudy region divides that higher part of the air, where the rain and similar things are generated, from the lower part, which is connected with the water and included under that name.