Before St. Thomas Aquinas, who laid the foundation for the existence of God?

John 10:24-33 (NIV)

The Jews who were there gathered around him, saying, “How long will you keep us in suspense? If you are the Messiah, tell us plainly.” Jesus answered, “I did tell you, but you do not believe. The works I do in my Father’s name testify about me, but you do not believe because you are not my sheep. My sheep listen to my voice; I know them, and they follow me. I give them eternal life, and they shall never perish; no one will snatch them out of my hand. My Father, who has given them to me, is greater than all; no one can snatch them out of my Father’s hand. I and the Father are one.” Again his Jewish opponents picked up stones to stone him, but Jesus said to them, “I have shown you many good works from the Father. For which of these do you stone me?” “We are not stoning you for any good work,” they replied, “but for blasphemy, because you, a mere man, claim to be God.”

Jesus did not come to make some apologetic works to proof the existence of God. Jesus came to prove that He is God and to prove the power of God to the people.

Jesus answered: "Don't you know me, Philip, even after I have been among you such a long time? Anyone who has seen me has seen the Father. How can you say, 'Show us the Father'? (John 14:9, NIV)

The foundation that God existed was laid long ago as men pondered their existence and place in the universe. This was expanded and reinforced by continuous prophetic utterances over the centuries that have been fulfilled. By the time of Christ’s ministry, the Jews were well grounded in the fact of God’s existence. Here are three foundation blocks.

As you will read, each argument moves from a wide focus into a narrow focus with each dependent yet independent of the other. God exists and extends Himself to us via nature, prophecy, and His Son. Each time God narrows the focus. Consider the creation, but do not worship it. Prophets speak for Me, but if their prophecy fails or leads you away, do not fall away. My Son fulfilled the prophets, by Him were the worlds created, if you do not have the Son, you do not have the Father.

ARGUMENT #1

Job is considered by most to be the oldest book in the Bible. He would be the first to lay out a foundation block. Job pondered the visible and knew there must be the Invisible above it who made it. Creation demands a Creator.

Job 31:26-28 [I]f I looked at the sun when it was shining, and the moon advancing as a precious thing, so that my heart was secretly enticed, and my hand threw them a kiss from my mouth, then this also would be iniquity to be judged, for I would have been false to God above.

This powerful argument was brought up in the psalms; for example in this one attributed to Moses.

Psalm 90:2 Even before the mountains came into existence, or you brought the world into being, you were the eternal God.

This same argument that the visible requires the Invisible was picked up some 2,000 years later by Paul.

Romans 1:20 For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:

Creation demands a Creator. We are without excuse. And then Paul relates this fact of seeing and not believing or perhaps worse worshipping creation and uses it against those who deny God or worship the creation rather than the Creator, the very thing Job had advised against.

Rom. 1:21-23 Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened. Professing themselves to be wise, they became fools, And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things.

So, in this argument we find that the creation demands a Creator. Indeed, this is God’s unanswerable argument of His existence.

ARGUMENT #2

Yet of course, people will try to explain away the Creator. So the next argument was established by the prophets of old. The argument is can man know the end from the beginning? The answer is we cannot. Yet God knows and He reveals it to His prophets. He spoke to the prophets. He defined how we would know God is speaking to us.

Num. 12:6 The LORD said, "Hear now my words: If there is a prophet among you, I the LORD will make myself known to him in a vision; I will speak with him in a dream.

Deut. 18:18 I will raise up a prophet like you for them from among their fellow Israelites. I will put my words in his mouth and he will speak to them whatever I command.

And again, this argument for God is brought up some 2,000 years later in the Book of Hebrews. God exists. He speaks to His people. We know it is God because what He says will come to pass does in fact come to pass.

Heb 1:1 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets,

God spoke to Adam (Gen. 3:9), Noah (Gen. 6:13), Abraham (Gen. 12:1), Moses (Ex. 3:4). As well as the major and minor prophets.

Paul also uses this argument to prove God’s existence.

Rom. 3:1 Much every way: chiefly, because that unto them were committed the oracles of God.

Moreover, man was instructed to write these things down as a witness. There are numerous examples: Ex. 17:14, Deut. 31:24, Hab. 2:2, Mal. 3:16. God wants to be known by His creation!

ARGUMENT #3

Still later, God sent His Son. Again, this was prophesied; it was predicted; and then it was fulfilled. And we see the image of God in the Son. This too proves God exists.

Heb 1:2 Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds;

This was made visible 2,000 years ago, written down, and still is known today.

Luke 24:44And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.

John 1:45 Philip findeth Nathanael, and saith unto him, We have found him, of whom Moses in the law, and the prophets, did write, Jesus of Nazareth, the son of Joseph.

This argument builds on the first and second arguments; that God exists and He has revealed Himself not only in nature, but by people who would truly prophesy, and by His Son.

CONCLUSION

God speaks to us in His creation. God speaks to us through His prophets. God speaks to us through His Son. The foundations for the existence of God were laid long ago. Look up. Our Redeemer draws nigh.

The most generic Christian answer to this is quite simple: God did.

No human—neither Aquinas nor any other philosopher or theologian—"laid the foundation for the existence of God". God has always existed and the impetus for revealing this truth to men has always been on him. This he has done in spades.

• Adam, the first man, walked and talked with God in a very tangible sense even before his wife Eve was created (see Genesis 2). Adam had no question about the existence of God or miss-understanding that he was God's creation.

• Throughout the Old Testament we see a God who was repeatedly in contact with his creation making himself known as creator and sovereign God.

  Hebrews 1:1 (ESV)
  ¹ Long ago, at many times and in many ways, God spoke to our fathers by the prophets,

• The Bible is blatantly forward on the issue of the existence of God being clear to all men through general revelation without the help of philosophers or theologians.

  Romans 1:20 (ESV)
  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.

This is not to devalue the contributions of Aquinas and others in helping fallible humans to get over themselves, out of the nonsensical funk we work ourselves up into and remind us of immanent truth. There is a time and a place for that. But your question mistakes the nature of their activity. Man did not create God, nor does he "lay a foundation for his existence". God is. And according to Christianity it is incumbent on all men to decide whether they will obey or disobey him.

Jesus' contemporary followers not only believed in "a god", they had a common understanding of the existence and nature of God, because his mission was to the people of Israel (cf. Jesus answer to the Canaanite woman in Matthew 15:14: "I was sent only to the lost sheep of the House of Israel"). This is not to assert that all of Jesus' followers came from the House of Israel, but before the writings of Paul, we have little evidence of any who were not. With the common understandings, they did not need an explanation of the existence, or nature of God.

If, on the other hand, Jesus had been sent to the Gentiles--the Romans, Egyptians, Greeks, &c.--he would have had to expend considerably more time teaching about the nature--though not the existence--of God. Note, though, that in the ancient world, atheism was rather unknown. Most everyone seems to have believed in at least one god, though there was not much consensus about much more than the existence of a god.