Titus Flavius Clemens, also known as Clement of Alexandria (c.150 c.215 AD), was a Christian theologian and philosopher who taught at the Catechetical School of Alexandria. Among his pupils were Origen and Alexander of Jerusalem. Clement is venerated in Coptic, Eastern, and Ethiopian Christian Orthodoxy. In his book, “The Stromata” written c.198-203 AD, he references the seven firmaments of the heavens.
“And they purify themselves seven days, the period in which Creation was consummated. For on the seventh day the rest is celebrated; and on the eighth he brings a propitiation, as is written in Ezekiel, according to which propitiation the promise is to be received. And the perfect propitiation, I take it, is that propitious faith in the Gospel which is by the law and the prophets, and the purity which shows itself in universal obedience, with the abandonment of the things of the world; in order to that grateful surrender of the tabernacle, which results from the enjoyment of the soul. Whether, then, the time be that which through the seven periods enumerated returns to the chiefest rest, or the seven heavens, which some reckon one above the other;”
– Clement of Alexandria [c. 150 – c. 215 AD]
(Stromata, Book 4, Chapter 25, pp. 877-878)