Given the efforts Aristotle made to support his argument for an eternal universe, it would appear that he was quite certain of the belief. He establishes multiple principles and arguments in making his case for an eternal universe (many of which creationists have gone to some lengths to either reconcile with creationism, or refute). Thus, many of those who read his works in medieval times did take the writings with a proverbial grain of salt, often deferring judgment on Aristotle to religious authorities (either people or texts).
I'll try to outline Aristotle's arguments below (thanks to Joseph for that link) to show that he indeed must have been quite certain of his belief. I'll also give some examples of responses from creationist philosophers, hopefully to indicate that most of them do not try to argue about Aristotle's own beliefs, but rather how they can be reconciled or dismissed to allow for creationism. Consequently, I answer that the common belief (supported by the contemporary SEP) is that Aristotle did hold his own views to be well establish.
Argument from Matter
In his Physics, Aristotle discusses his view of matter extensively. As a part of this discussion, he makes an argument in support of eternal matter. Aristotle asserts that all things must come into existence from an underlying "substratum", which is a sort of essence of being. Then he argues that matter itself (the Aristotelian concept of matter) is the substratum of all things, so it must have either created itself, or been eternal. Obviously, Aristotle argues, a substratum cannot create itself; therefore, matter must be eternal.
Argument from motion
To Aristotle, the process of coming into existence is a sort of motion, and motion is a very important concept to Aristotelian philosophy. Aristotle's argument is that if a beginning to motion (i.e. existence) is to be assumed, one of the following must be true:
- The object that experienced this first motion must have come into existence and moved.
- The object that experienced this first motion must have existed at rest, and then moved.
Aristotle argues that #1 is contradictory, because the object could not have already been moving before coming into existence, and in order to have moved, it requires a preceding motion: that of coming into existence.
He further argues that #2 is flawed because the existence of the object implies that it came into being, which is already a motion. Furthermore, in order to cause the first motion, a preceding motion is required, which is a sort of infinite regress.
Because the idea of a first motion results in these unsatisfactory conclusions, Aristotle asserts that motion must have been eternal.
Argument from Time
Here, Aristotle employs contradictions to support his argument. His argument is that if time had a beginning, then one could refer to a time before that beginning (because something must have preceded a beginning), and this is contradictory to the premise. Furthermore, to Aristotle, time is a series of beginnings and ends, or moments; each moment ends the one before it. Thus, a "first moment" or beginning to time must have ended a moment before it, and this too is contradictory.
Horror Vacui
Nature abhors a vacuum. This famous and extremely influential statement is the basis for Aristotle's argument "from the vacuum." He argues that a vacuum cannot exist; nature will do anything to avoid one. Therefore, to imply a beginning of the universe is to go against this fundamental principle, for before the beginning there must have been nothingness, or a vacuum.
Of course, the very concept that nature abhors vacuums has since been empirically refuted.
Response from Thomas Aquinas
Thomas Aquinas was a very influential Medieval Christian Philosopher, and played a big role in reintroducing Aristotelian philosophy. He wrote many commentaries on Aristotle's works (such as the metaphysics), and adopted much of Aristotle's philosophy. However, the SEP makes the following important note:
Given the distinction between philosophy and theology, one can then distinguish between philosophical and theological sources and influences in Aquinas' work. And as a philosopher, Thomas is emphatically Aristotelian.
However, in his theology, Aquinas is very Christian. Although he took much from Aristotelian philosophy, he did not attempt to combine it with Christian theology, and the concept of creationism. The SEP points out that his philosophical acceptance of Aristotle does not mean that "he never disagrees with his sources." The eternity of the universe is one of these points of disagreement; Aquinas takes Aristotle's view very seriously, but simply disagrees with it. He does not attempt to argue that Aristotle saw his own philosophy of eternity as somehow flawed.
Response from Al-Farabi
Al-Farabi, one of the first major Muslim philosophers, was a scholar of Aristotle. However, in his philosophical writings (obviously influenced, and actually motivated, by Islam) he "breaks with the philosophy of Plato and Aristotle." He too takes Aristotle seriously, but he attempts to reconcile his (and Plato's) philosophy with a First Cause (a principle of Islam), "using him as an authority when it suited [his] purposes, and knowing that philosophy was a 'foreign science' in need of an external authority as it lacked an indigenous authority"(source).
Al-Farabi, having drawn from not just Aristotle, but Plato and even Ptolemy in his metaphysics, does use Aristotle's principles of causation. However, equally important to him is the concept of a First Cause. Thus, Al-Farabi considers Aristotle arguments to represent his beliefs, but simply does not agree with some of them. This view is similarly held by the majority of medieval Islamic philosophers.
Summary
Aristotle's efforts to support his idea of an eternal universe, not created ex nihilo, indicate a strong belief in what he is trying to prove; if he saw his arguments as flawed, why would he attempt to convey them?
The medieval responses act as the "classic commentators" you ask for in your question; although they do not directly respond, they do indicate that the classic (perhaps, medieval) view of Aristotle held that he believed his own philosophy to be true; thus, they simply make a point to disagree with him.
As for a contemporary view, the SEP is always reliable, and offers no indication that there is a mainstream questioning of Aristotle's own beliefs about the eternity of the universe. Rather, he went to great lengths to establish and support them, and they are about as important to him as his other principles. Unfortunately, I cannot offer specific commentaries, but the difficulty in finding any that question Aristotle's personal beliefs speaks to my argument.
