According to Catholicism, is the existence of God self-evident? What do Catholic theologians say regarding God’s existence being self-evident?
You're not going to find one answer on this. To some, the existence of God is self-evident, to others it's not. Then you get into "what does self-evident mean".
Some of those who say God is self-evident claim so because the complexity of creation screams "an intelligent designer made me". But that's not self-evident, it's evidence from creation.
Others claim Romans 1 and 2 - the existence of God is clearly known because He wrote it on our hearts, but that's not self-evident, either. The may point to 2 Peter chapter 3 as support that we know God, but are "willingly ignorant" because we'd rather walk after our own lusts, but that's not "self-evidence" either.
St. Thomas Aquinas addressed the question is his Summa Teologica, however, and gives a good answer (that also meets your requested criteria for a quote from a reputed theologian). emphasis mine in the passage below
He does an excellent job of defining what self-evidence is, and therefore showing how the existence of God meets (or doesn't meet) those definitions.
Whether the Existence of God Is Self-Evident?
We proceed thus to the First Article:—Objection 1. It seems that the existence of God is self-evident. Now those things are said to be self-evident to us the knowledge of which is naturally implanted in us, as we can see in regard to first principles. But as Damascene says (De Fid. Orth. i. 1, 3), the knowledge of God is naturally implanted in all. Therefore the existence of God is self-evident.
Obj. 2. Further, those things are said to be self-evident which are known as soon as the terms are known, which the Philosopher (1 Poster. iii) says is true of the first principles of demonstration. Thus, when the nature of a whole and of a part is known, it is at once recognized that every whole is greater than its part. But as soon as the signification of the word "God" is understood, it is at once seen that God exists. For by this word is signified that thing than which nothing greater can be conceived. But that which exists actually and mentally is greater than that which exists only mentally. Therefore, since as soon as the word "God" is understood it exists mentally, it also follows that it exists actually. Therefore the proposition "God exists" is self-evident. ** Obj. 3. Further, the existence of truth is self-evident. For whoever denies the existence of truth grants that truth does not exist: and, if truth does not exist, then the proposition "Truth does not exist" is true: and if there is anything true, there must be truth. But God is truth itself: I am the way, the truth, and the life (John xiv. 6). **Therefore "God exists" is self-evident.
On the contrary, No one can mentally admit the opposite of what is self-evident; as the Philosopher (Metaph. iv., lect. vi) states concerning the first principles of demonstration. But the opposite of the proposition "God is" can be mentally admitted: The fool said in his heart, There is no God (Ps. lii. 1). Therefore, that God exists is not self-evident.
I answer that, A thing can be self-evident in either of two ways; on the one hand, self-evident in itself, though not to us; on the other, self-evident in itself, and to us. A proposition is self-evident because the predicate is included in the essence of the subject, as "Man is an animal," for animal is contained in the essence of man. If, therefore the essence of the predicate and subject be known to all, the proposition will be self-evident to all; as is clear with regard to the first principles of demonstration, the terms of which are common things that no one is ignorant of, such as being and non-being, whole and part, and such like. If, however, there are some to whom the essence of the predicate and subject is unknown, the proposition will be self-evident in itself, but not to those who do not know the meaning of the predicate and subject of the proposition. Therefore, it happens, as Boethius says (Hebdom., the title of which is: "Whether all that is, is good"), "that there are some mental concepts self-evident only to the learned, as that incorporeal substances are not in space." Therefore I say that this proposition, "God exists," of itself is self-evident, for the predicate is the same as the subject; because God is His own existence as will be hereafter shown (Q. 3, A. 4). Now because we do not know the essence of God, the proposition is not self-evident to us; but needs to be, demonstrated by things that are more known to us, though less known in their nature—namely, by effects.
Reply Obj. 1. To know that God exists in a general and confused way is implanted in us by nature, inasmuch as God is man's beatitude. For man naturally desires happiness and what is naturally desired by man must be naturally known to him. This, however, is not to know absolutely that God exists; just as to know that someone is approaching is not the same as to know that Peter is approaching, even though it is Peter who is approaching; for many there are who imagine that man's perfect good which is happiness, consists in riches, and others in pleasures, and others in something else.
Reply Obj. 2. Perhaps not everyone who hears this word "God" understands it to signify something than which nothing greater can be thought, seeing that some have believed God to be a body. Yet, granted that everyone understands that by this word "God" is signified something than which nothing greater can be thought, nevertheless, it does not therefore follow that he understands that what the word signifies exists actually, but only that it exists mentally. Nor can it be argued that it actually exists, unless it be admitted that there actually exists something than which nothing greater can be thought; and this precisely is not admitted by those who hold that God does not exist.
Reply Obj. 3. The existence of truth in general is self-evident but the existence of a Primal Truth is not self-evident to us.
From a Christian perspective, yes, the existence of God is self-evidence. Paul teaches in the book of Romans about this:
(I understand that atheists may take offense to this, but none is intended. This is just the teaching of the Bible.)
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth. 19 For what can be known about God is plain to them, because God has shown it to them. 20 For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. 21 For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. 22 Claiming to be wise, they became fools, 23 and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things. Romans 1:18-23 ESV
Several key points here:
- The first step was to suppress the truth.
- What can be known about God is plain to them,
- God has shown mankind the things that can be known about God.
- God's invisible qualities are clearly perceived in what has been made.
- Mankind is without excuse.
- The knew God (past tense), but in their denial of God, they became fools.
So, the existence of God is, indeed, self-evident. However, people can suppress the truth and live in denial of that reality, even vehemently protesting that there is not sufficient evidence for God.
A few other verses also support this:
He has made everything beautiful in its time. Also, he has put eternity into man's heart, yet so that he cannot find out what God has done from the beginning to the end. Ecclesiastes 3:11 ESV
The heavens declare the glory of God, and the sky above proclaims his handiwork. 2 Day to day pours out speech, and night to night reveals knowledge. 3 There is no speech, nor are there words, whose voice is not heard. 4 Their voice goes out through all the earth, and their words to the end of the world. Psalm 19:1-4 ESV