According to the First Vatican Council  (1869-70), it is a de fide dogma of the Catholic Church “that there is one true and living God, creator and lord of heaven and earth, almighty, eternal, immeasurable, incomprehensible, infinite in will, understanding and every perfection.”
It is important to remind ourselves of this truth, since there are many in the religious world, including those in non-Catholic Christian communities, who have rejected important aspects of it, as I have already noted on this page .
The Church’s belief about God starts with creation. God is Creator of all that exists. (Gen 1:1; Ps 33:8-9; Ps 124:8; Ps 146:5-6) Yet for the Catholic, God is not merely a Divine Craftsman who works with pre-existent eternal matter, but is the source on which all contingent reality, including matter, depends for its existence. (Acts 17:25: Col 1:16-17)
Because God is the Source of all contingent reality, and thus not himself a contingent reality, he must by nature be self-existent, meaning he has the attribute of aseity. In short, God exists necessarily. Unlike the universe and everything in it, nothing is required for God to exist. He simply IS. Thus, when Moses encountered God in the burning bush and asked God to identify himself:
God said to Moses, “I am who I am.” He said further, “Thus you shall say to the Israelites, ‘I am has sent me to you.”’ (Ex 3:14)
He is, in other words, the self-existent one. This is clearly implied not only in passages where we are told God is unlimited in his power (Jb 42:2; Je 32:17; Mt 19:26), but also when Scripture tells us, by the use of metaphorical language, that God’s nature is to always be. (Is 41:4; 43:10 44:6b)
As Creator and Sustainer of all that exists, God is the First Cause. By “first cause” we do not mean that God is “first” in a temporal series of causes. Rather, we mean that he is the First Cause metaphysically. That is, God is first in a foundational sense. The world’s causal activity is contingent and conditioned; it cannot contain within itself an explanation for its own existence.
For this reason, the causal activity we observe must be caused by – or grounded in – that which itself is necessary and unconditioned. Some may insist that we need not explain the causal activity we observe by appealing to anything other than the proximate causal forces that precede that activity. For example, one need not explain my coming into being by appealing to anything other than the procreative act of my parents. Any other explanation is thought, by some, to be superfluous. This response, however, confuses two ways of conceiving causes and the causal series in which they occur.
The difference is whether there must exist a First Cause (or uncaused cause) of a causal series per accidens (i.e., Can the present be the result of an infinite past series of events?), or there must exist a First Cause (or uncaused cause) of a causal series per se. (Can a present event, X, i.e., be the result of an infinite series of causes right now?)
For Catholics, God keeps the universe in existence at every moment, since a universe, even an everlasting and infinitely large one, consisting entirely of contingent beings in causal relationships with each other, could no more exist without some sustaining First Cause than could an alleged perpetual motion machine exist without an Unmoved Mover keeping its motion perpetual.
Here’s an illustration adapted from an example employed by one of my former professors, the late James Sadowsky, S.J.  Imagine the universe consisted entirely and exclusively of people who could not act without first asking permission from another person, and suppose that this universe had always existed. Imagine further that everyone is acting. Unless there is a First Giver of Permission (who by nature does not have to ask permission in order to act) outside the causal series of permitters and actors, no one would ever act, since asking permission is itself an act for which one would need permission.
For these reasons, the Catholic maintains that it is wrong to think of God as if he were one incredibly great being (or the Greatest Being) – as some Christians seem to teach – but rather, we should think of God as the Being that gives existence but does not receive it.
This is what St. Paul seems to have had in mind when he preached at the Areopagus:
The God who made the world and everything in it, he who is Lord of heaven and earth, does not live in shrines made by human hands, nor is he served by human hands, as though he needed anything, since he himself gives to all mortals life and breath and all things. From one ancestor he made all nations to inhabit the whole earth, and he allotted the times of their existence and the boundaries of the places where they would live, so that they would search for God and perhaps grope for him and find him – though indeed he is not far from each one of us. For “In him we live and move and have our being”; as even some of your own poets have said, “For we too are his offspring.” (Acts 17:24-28, emphasis added)
St. Paul is making a philosophical claim about God as the sustaining and transcendent, though immanent, First Cause of all that exists (“The God who made the world and everything in it,” “In him we live and move have our being”), while quickly adapting this philosophical claim to anthropomorphic poetic language that his audience would understand: “For we too are his offspring.”
There is, of course, more to the Catholic understanding of God than this, including the doctrine of the Trinity . But we first have to get creation right, since  “a small error at the outset can lead to great errors in the final conclusions.”