Paraphrasing the Summa: First part, Question 2, Article 1

Question 2. The existence of God

Does God exist?  In article 1, below, St. Thomas doesn't answer this question.  He explains why some people do and some people do not believe in God.  

Article 1. Whether the existence of God is self-evident?

Does God's being need to be proven?

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.

First principles?  Have you ever heard someone say, "that's a given."  That means that it goes without saying.  Or that it is common knowledge to all.  Remember in grade school when they spoke of the "commutative principle" of addition?  That is a first principle of addition.  This is objector is talking about the first principles of proving something in philosophy.  That is what this objection is talking about.  And he says that the existence of God is a first principle, such as this.

But as Damascene says (De Fide Orth. i, 1,3), "the knowledge of God is naturally implanted in all." Therefore the existence of God is self-evident.

He says that St. John of Damascus says that the knowledge of God is in all men, therefore, the existence of God is self evident and does not need to be proven.

Objection 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.

Aristotle, the Philospher, said that those things are obvious which are recognized as soon as they are named.

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.

So, if we speak of a car and we speak of a car's headlight, we right away know that the car is much bigger than the headlight because the headlight is a part of the car.  This is obvious to all.

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.

Everyone understands the word "God" to represent the being who is greater than anything which exists.  Therefore, God exists.

But that which exists actually and mentally is greater than that which exists only mentally.

 And that which exists in real life as well as in the mind is greater than that which exists only mentally.  (Compare this to the whole is greater than the part, above).

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.

So, since, as soon as God exists in the mind, He also exists in reality, then it is obvious that God actually exists.

Objection 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 14:6) Therefore "God exists" is self-evident.

No one can deny that truth exists.  Because, if one claims that the idea"truth does not exist" is true, then it there is at least one truth. And truth does exist.  And since God is truth, as Scripture says, "I am the way, the truth and the life".  Then, it is obvious that God exists.

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" (Psalm 52:1). Therefore, that God exists is not self-evident.

 St.  Thomas argues against the idea that God's existence does not require proving.  Since atheists exist, he says,  and they deny the existence of God. That is proof that the existence of God is not obvious to all.

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.

Huh?  I think he means that something can be self evident to some, but not to others.  But, we'll keep reading.

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.

Some ideas are obvious.  For example.  If we say that "a dog is an animal".  Those who have seen dogs, know that dogs are animals before they are told.  So, this is obvious.

If, however, there are some to whom the essence of the predicate and subject is unknown,

But, if someone has had no experience with dogs.  That person would not know what you were talking about when you say, "dog".  Therefore, for them, this idea of "a dog is an animal" is not obvious, since they have never heard of a dog.

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.

So, if the person is ignorant of a subject, to that person, the notion being discussed is not obvious.  *

* We have an example of this with native Americans.  It is said that when they first saw horses, they assumed they were giant dogs.  Whereas, to a European, the idea of a horse being an equine was self evident, to a native American, it was not. 

Therefore, it happens, as Boethius says

Boethius is an early Church philosopher and martyr.

(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."

Now, this I understand.  Boethius says that some concepts are self-evident only to those who have studied the idea (the learned).

Therefore I say that this proposition, "God exists," of itself is self-evident,

So, then, to those who believe in God, the idea that God is real, is obvious.

for the predicate is the same as the subject, because God is His own existence as will be hereafter shown (3, 4).

??What the predicate and the subject have to do with it, I have no idea.

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.

Because God's nature can not be known by the human mind, it is only by studying His creation that we can come close to understanding the nature of God.  So, the existence of God is not obvious.

Reply to Objection 1. To know that God exists in a general and confused way is implanted in us by nature,

In general, men know that something exists which is greater than us.  But that is a vague idea of God.

 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.    

But all men desire to be  happy.  And all men desire to know God because to know God is to be happy.

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.

But many people mistake creatures and comforts for the ultimate good which is God.

Reply to Objection 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.

Not everyone understands the significance of the word, "God".

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 does everyone who understands the word, believe it signifies anything real.

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.

And just because some believe that something exists, that is not proof of its existence.  That is why some do not believe that God exists.

Reply to Objection 3. The existence of truth in general is self-evident but the existence of a Primal Truth is not self-evident to us.

The fact that truth exists, is self evident.  But not all truths are self evident.