With objections answered, the First Cause Argument is indeed valid.
|A. Anything that has a beginning has a cause.
B. Physical reality had a beginning.
A+B-->C. Physical reality has a cause.
Said cause, that which can create physical reality where otherwise there would be none, is commonly called God or Creator.
Objection 1: "What if the Universe had no beginning and has just always been?"
That was the Static State Universe. It has been debunked ever since the Big Bang Background was detected in 1965.
Objection 2: "Couldn't the Multiverse be infinitely old?"
As explained in "A Commentary on God and the Multiverse," only the Level IV Ultimate Ensemble is without a beginning. The Level IV Ultimate Ensemble, however, consists of potentiality rather than actuality, making it insufficient to explain the actualities we observe.
Objection 3: "What about cyclical Big Bangs and Big Crunches in the past?"
Such models are subject to meta-expansion. Each cycle would be larger, at its height, than the cycle preceding it. Tracing into the past, said maximum would eventually reach 0, thereby constituting an absolute beginning.
Objection 4: "What about the Quantum Static Seed?"
Quantum Mechanics itself shows such models to be past-finite. Here is how.
If the Static Seed were infinitely old, it should have collapsed into the Semi-Classical expanding relativistic state an infinite time ago. As long as such collapse is possible in the first place, that is. If such a collapse is impossible, the Universe should remain a Static Seed now and forever.
These paradoxes are only avoided by the Static Seed and any associated Quantum Vacuum having a beginning. A finite age makes these models work.
Objection 5: "What about a Quantum Oscillation?"
This is a tiny Cyclical Universe, a combination of the models referenced in Objections 3 and 4. Such an early Universe may well have existed, but if so it was past-finite (had a beginning) for the same reasons likewise combined.
Objection 6: "That argument only concludes in a God of some kind. It does not argue for Christianity over other religions."
Of course not, and no one made that claim. No one claims that the First Cause Argument, by itself, specifically supports Christianity over any other religion and any other description of God.
In other words, the objection is an off-topic red herring. To support Christianity more specifically, one must discuss the historicity of Jesus and related matters. Those are entirely different topics.
Objection 7: "The series of negative integers has no first member. So why should time or causality?"
Although they proceed in opposite directions, the positive and negative integers nevertheless proceed from the same central starting point. That starting point is 0, and from it numbers proceed in positive and negative directions away from each other.
Alternatively, in the example of pure mathematics, the negative numbers (and indeed, even 0) are derived from the natural integers (1, 2, 3, and so forth infinitely) via the operation of subtraction. Moving from the discrete infinite series to a continuous one, all fractions (non-integer numbers) are likewise derived from integers via the operation of division.
A more tangible form of this objection, using physics rather than pure mathematics, would be Guth's Time Arrow Reversal. If Time itself flows in the opposite direction "before" the Big Bang, however, than by definition it is no longer a past we are dealing with, but rather an alternate future. This can be extended into a Multiverse argument, whereby if 2 timelines can start at the same Big Bang, any number of other timelines can also start there.
Objection 8: "The Leibnizian Principle of Sufficient Reason, while very much at home in Classical Physics, has no obvious place in Quantum Mechanics."
This objection derives from a conflation of different meanings of "nothing" or "nothingness." Per Quantum Mechanics, particles do indeed appear from empty space on a regular basis. That empty space, however, is not Nothingness in the absolute philosophical sense (nor indeed is any empty space, even by Classical understanding). On the contrary, the Quantum vacuum is a space-time with both a fundamental fine structure and various universal force fields.
Objection 9: "Why rely on an argument like this? Aren't the Bible, the Early Church Fathers, and the Doctors of the Church good enough?"
Using the Bible to argue the existence of God would be circular. Furthermore, Doctor of the Church Saint Thomas Aquinas was a pioneer who wrote a remarkably impressive version of the First Cause Argument, for someone writing 7 centuries before Big Bang Theory became available!
In fact, he was declared a Doctor of the Church for 2 reasons, this being one of them. The other was his discourse on the Baptism of Desire and conversion at the instant of physical death. Earlier authors had discussed that topic in passing, but Aquinas was the first to clarify it in detail.
Objection 10: "Just because there is an intangible First Cause, that doesn't mean there's a sky fairy! Why would the First Cause choose to create a sky fairy but no evidence for him?"
There is no sky fairy. This objection is simply a straw man. The Church understands God to be the First Cause, not a sky fairy.
Quod Erat Demonstrandum.