And the likeness of the firmament upon the heads of the living
This was not a real firmament, but the likeness of one; it was like the firmament which God created on the second day, which divided between the waters above and below, and which he called Heaven; and is no other than the airy and starry heavens. Its name is in Hebrew (eyqr) , "an expanse"; from its being stretched out as a curtain, and a tent to dwell in; and a "firmament" from its firmness and continuance; and therefore called the firmament of his power, ( Genesis 1:8 ) ( Isaiah 40:22 ) ( Psalms 150:1 ) ; and this visionary one was "over the heads of the living creatures"; which shows that they could not be angels, for those have their habitation in the third heaven, above the firmament; much less in hieroglyphic of the trinity of Persons, who are the three that bear record in heaven, and are not under the firmament; but ministers of the Gospel, who are on earth, and are subject to Christ, whose throne is above the firmament, ( Ezekiel 1:26 ) ; and who receive their commission and gifts from him, and are accountable to him. This firmament [was] as the colony of the tenable crystal;
crystal is a very white, transparent, precious stone, resembling ice, from whence it has its name; hence Pliny F20 thought it was no other than ice vehemently frozen; and here it is called "terrible", because exceeding clear and bright, so that there was no looking upon it, without the eyes being dazzled with the glory of it. The sky is called a molten looking glass, in which the glory of God, and his handiwork, may be seen, ( Job 37:18 ) ( Psalms 19:1 ) ; and as the throne of Christ was over this crystal firmament, it shows that, though he is in heaven, he sees all that is done on earth, and in his churches, and by his ministers; and the saints also see him by faith, and through the glass of the Gospel: it is only a crystal firmament that is between them, stretched forth over their heads above;
that is, over the heads of the living creatures, as before; said to be stretched out, in allusion to its name, an expanse, as before observed.