A. P. Lopukhin. Explanatory Bible (Saint-Petersburg, 1904-1913):

For the almighty Creator of the universe, a thought or a word and the realization of this thought, or deed, are completely identical with each other, since for Him there are no obstacles that could prevent the fulfillment of a born volition. Hence, His word is the law for being: “For he spake, and it was done; he commanded, and it stood fast.” (Ps 33:9). Following many Church Fathers, Met. Philaret believes that in the word “said”, not without reason, one can find the mystery of the Hypostatic Word, which here, just as the Holy Spirit previously was, - is covertly presented as the Creator of the world: "this interpretation is supported by David and Solomon, who obviously adapt their expressions to Moses"  (Ps 33:6; Proverbs 8:22-29).

“Let there be light…” Apostle Paul gives a clear indication of this, speaking of God as of One “who commanded light to shine out of darkness” (2 Corinthians 4.6). The creation of light was the first creative and formative act of the divine creation of the universe. This primordial light was not ordinary light in the usual sense of the word, since before the fourth day of creation, on which the night luminaries appeared, there were no sources of our light yet. Rather, it was that luminiferous ether, which, being in an oscillatory state, dispersed the primordial darkness and thereby created the necessary conditions for the future appearance of any organic life on earth.