As we prepare for our annual commenoration and celebration of the Incarnation of our Lord, Jesus Christ during this Advent, 2021 season in the Church, let’s take a step back for a moment and reflect upon the the existence of God in the first place. Who better to address and analyze this profound area of philosophical and theological inquiry than Saint Thomas Aquinas; in this case in his analysis and arguments contained in Summa Theologiæ I.1

Reading and reflecting on this central issue in our faith as Catholics, Aquinas proceeds from a basic notion that while God is beyond our rational comprehension, sacred doctrine is devoted to transmit knowledge of the divine. In other words, despite the infinite chasm between God and man, there is such a thing as theological methodology that can be addressed to God’s existence not only as a function of revealed truth but also as a function of natural human reason. Aquinas also argues that our limited human powers allow for us to doubt the Primal Truth of God’s existence. While the existence of God may in itself be self-evident as a truth – as His essence is to exist – such existence may not be axiomatically known to us through our own limited human powers. Rather, our intellectual powers are a proportional participation in the divine intellect. As Aquinas notes, “the existence of truth in general is self-evident but the existence of a Primal Truth is not self-evident to us.”2 According to Aquinas something can be self-evident in one of two ways. First, something can be self-evident in itself but not to us; and second in both itself and to us. A key point in this distinction is that while it is self-evident to God that He exists (no human reasoning or conclusions are required), it is most definitely not self-evident to us that God exists. Rather, we require reasoned evidence, and therefore we must prove the existence of God. 3

Aquinas emphasizes in this domain that God’s existence is God’s essence, and we can demonstrate God’s existence from the effects of God, even though from these effects we cannot really know the essence of God. As such, the manner in which to demonstrate the existence of God lies in employing a posteriori reasoning rather than a priori reasoning. This is because any demonstration of God’s existence must proceed from the effects of God known to us. In other words we can use reason to conclude that God exists because of the effects of God, and then reason back to that which produced those effects (a posteriori). We cannot, however, first know in a self-evident way that God exists and then work out what the effects of God’s existence should or must be (a priori).4 Further support concerning the existence of God via a posteriori reasoning is also seen in the Church’s Dei Verbum. An example of knowing that God exists through such a posteriori reasoning is found when the Church promulgates that “in His goodnessand wisdom God chose to reveal Himself and to make known to us the hidden purpose of His will (see Eph. 1:9) by which through Christ, the Word made flesh, man might in the Holy Spirit have access to the Father and come to share in the divine nature.”5 It is in this way that we come to know through these and other effects that God exists.

Further, according to Aquinas the existence of God can be proved in five ways. First, God is the primary mover who put everything into motion without Himself being moved: “It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act...Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.”6 Put directly, since nothing can move itself it must be put into motion by something else, otherwise there would be no motion in the first instance. Since there exists such motion there must be a first mover, and that is God. Second, by way of efficient cause we can say that nothing causes itself. Efficient causes come from the first cause through intermediary causes to the ultimate effect. Since nothing can be the cause of itself, but there are such effects, “it is necessary to admit a first efficient cause, to which everyone gives the name of God.”7

Third, Aquinas argues both possibility and necessity when he concludes that everything in nature is either necessary or contingent. If we assume that everything is the latter, it would not be possible for contingent beings to exist if at one time if nothing was in existence. Since we do exist, it would have been impossible for anything to come into existence. Therefore, “we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.”8 Fourth, with regard to scales of perfection, there is gradation found in the things of nature. In other words, things exist in proportion to that which is maximum or the most of something. This includes goodness and any other perfection. We encounter these scales of perfection by employing terms like “less” and “more” for example. According to Aquinas “there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God.”9 Finally, Aquinas argues by way of design, orderliness and governance that we observe that natural bodies act toward ends – through knowledge or by direction of something with knowledge. Since many things in nature lack such knowledge, “some intelligent being exists by whom all natural things are directed to their end; and this being we call God.” 10

As we celebrate our Savior’s birth, the Word Made Flesh, let us also share not only with our fellow Catholics, but with all whom we encounter in our travels in life, the great thought and scholarship of those scholastics such as Aquinas who have preceded us. In examining and reflecting upon the existence of God, we ultimately seek to prepare ourselves not only for Christ’s birth, but also His return in glory.