This is, surprisingly, a common objection to Aquinas’ cosmological argument. Those who raise it think they have hold of a logical proof that Aquinas’ argument is circular and self-defeating.
But it’s surprising that it is such a common objection because it in fact betrays a profound misunderstanding of Aquinas’ argument. You’ll find dozens of such objections on Youtube where unsuspecting critics blunder away at a version of the argument that is certainly not Aquinas’.
In fact, nowhere does Thomas Aquinas argue that “everything must have a cause.” In fact, he argues precisely the opposite—that everything cannot have a cause; there must be some being who is not caused, otherwise nothing would exist.
So to respond to this argument by saying, “Then what caused God?” is simply an indication that the interlocutor has missed the point. Aquinas’ Five Ways rest on the premise that there is some entity that is not caused.
So to put it simply: there are only three possibilities for our universe: (1) that it is eternal and that matter or parallel universes or whatever you like have always been around; (2) that the universe has not always been around, but it has popped into existence from nothing, spontaneously and without any outside help; (3) that there is some Uncaused Causer which is the cause of everything else.
The first three of Aquinas’ Five Ways in his Summa Theologica (Prima Pars, Q2, a3) aim to show that options (1) and (2) are not viable—which leaves us with option (3): there is an Unmoved Mover or First Cause behind everything else that exists.
Note too, that in demonstrating the necessity of an Uncaused Causer Aquinas is not claiming to have proven the existence of the God of Christianity or Judaism. Having demonstrated the necessity of an Uncaused Causer he adds—almost incidentally—that this entity is what Christians call God.
