How long has the universe existed? Has it been around forever? Did it have a beginning? Could it have a beginning? These questions troubled the ancients.
Aristotle in his Physics tried to answer this through reason. Everything that comes into being does so from something that already exists; matter is made from matter, after all. The matter of the universe would have to come into being from some underlying matter; it couldn't come from nothing. For the matter of the universe to come into being, some matter must have existed before it. This statement is ridiculously self-contradictory, and therefore could not be taken seriously. The universe must have always existed.
Others supported Aristotle. Critolaus (c.200-c.118 BCE) couldn't believe that human beings would ever stop simply procreating into eternity. In the Early Medieval Period, Proclus (412-485) produced De Æternitate Mundi (On the Eternity of the World) with 18 proofs.
This belief was about to collide headlong with Christianity, however. The Bible makes it clear that there was a moment of Creation. That being the case, the universe cannot have been eternal.
John Philoponus (490-570) was a prolific and controversial writer who realized that Aristotle needed to be questioned on some things. Although he would be condemned after his death as heretical for interpreting the Trinity as three gods instead of one God, he was known in his lifetime for defending the Biblical necessity of a universe with a starting point. He wrote "Against Proclus" in which he challenged every one of Proclus' arguments. The basis for his argument is simple, and referred to now as the Traversal of the Infinite. If the existence of something relies on the existence of something else prior to it, then you need to account for the existence of the prior thing. That prior thing would rely on the existence of something before that, and so on. You have to have an infinite series of assumptions that something existed before the thing that came afterward, and never actually explain where any of the substances came from. The world could not possibly be infinite, and must have been created by a divine being.