The Westminster Confession with its over 4000 direct quotes of the Scripture, chose not to leave creation to a direct quote in 4.1, but instead to interpret the creation. Not as the ancients in seeing creation as instantaneous; not as the medievals who saw adornments and literary features; but as Calvin would understand it–a literal six day creation.
The Westminster Confession makes over 4000 direct references to and quotations of the Scriptures. “In the space of six days” is not a reference to or quotation of the Scriptures.
The Westminster Confession of Faith 4.1 says: “It pleased God the Father, Son, and Holy Ghost, for the manifestation of the glory of His eternal power, wisdom, and goodness, in the beginning, to create, or make of nothing, the world, and all things therein whether visible or invisible, in the space of six days; and all very good.”
“In the space of six days” is not a biblical phrase, but a theological phrase, intended to interpret the Scriptures rather than quote them.
Basil, referencing Origen said, “This beginning was instantaneous… in a rapid and imperceptible moment.” Origen said the world as we know it was created in one day, but many of the ancients thought the world was eternal (including Origen). They reflect Plato who said that the world was created out of preexistent matter.
Augustine also believed in an instantaneous creation. He wrote, “[God] created all things simultaneously and also created this one day, seven times repeated. The need for these six days to be set out was to teach those who could not understand simultaneous creation…God accommodated himself to the capacity of weaker intellects and presented creation as if it were a process.” (Works, 4.51-52; 5.5)
Medieval thinkers followed suit with Augustine. Bede, Anselm, and even Aquinas followed Augustine. But Aquinas “distinguished” in scholastic format saying that the creation was three-fold and there was an adornment process that occurred.