Before St. Thomas Aquinas, who laid the foundation for the existence of God? During His preaching ministry, Jesus described the nature of God. But before doing so, did He have to lay a foundation and convince people that God exists? Or had oral tradition and the Old Testament already convinced most people that God exists?

Reason I ask:  


Christians typically credit Saint Thomas Aquinas' 'Quinque viae' as the best arguments for the existence of God.
But I assume that most of Jesus' contemporary followers must have already believed in "a god", because if they didn't then Jesus would have needed to spend a good deal of His effort persuading the masses that "a god" (that is any god) exists, as opposed to "the God" that He existed as in the Holy Trinity.
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, &amp;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.