Nothing besides God has existed always. Things exist inasmuch as God wills they should be, and He wills nothing of necessity except Himself. Hence the world was when God willed it to be; nor do the reasons of Aristotle prove absolutely that the world existed always; rather, they contravene certain opinions of the ancients, which supposed the world to have begun in ways incompatible with truth.
The beginning of the world we hold, however, by faith only, for its recent origin cannot be demonstrated either from itself, or from its Cause, which is God. For God acts by Will, and His Will cannot be investigated by reason except with regard to things which He wills of absolute necessity. Those which concern creatures are certainly not such; but the Divine Will may be known by revelation, and this is its conclusion.
That, "In the beginning God created the heavens and the earth," admits of a threefold explanation. It excludes, first, the error which denies that time had a beginning; and secondly, that which admits two principles, one good, the other evil; thus "in the beginning" is to be understood as in the Son, to Whom, in regard of Wisdom, the exemplary principle is attributed. Thirdly, it excludes the error which holds corporeal beings to have been created by God through the medium of spiritual ones. For four creations are generally held to have been simultaneous: the Empyreal Heavens; Matter, understood by earth; Time, and the Angelic Nature.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni