The Cosmological argument is the best of the 'proof of existence of God' outside faith arguments, there is however scientific wiggle room. The Cosmological Argument as proof of God was fine when the 'Big Bang' was the singular theory for the existence of the universe, but things are changing.
Astrophysics investigators asked exactly the same questions as the Cosmological Argument, but proposed very different hypotheses ... Thus
4 – The universe cannot be the cause of itself (from 3)
5 – Something outside the universe must have caused the universe (from 2 and 4)
6 – God is the only thing that is outside of the universe
One hypothesis forwarded is that there is a proto-universe from which all other universes form. I can't remember what they called this "proto-universe", but its the same concept. Thus,
6 – Either God or the proto-universe is the only thing that is outside of the universe
It would therefore answer the Cosmological Argument and exclude God. If there was evidence for a proto-universe in the future (its a mathematical theory), and its not inconceivable, then the Cosmological Argument would need to be revised to say:
4 - The proto-universe cannot be the cause of itself
The trendy theory right now is there was a universe before the Big Bang together with supporting evidence. That doesn't change the Cosmological Argument, but it does alter how it fits into scientific context.
For apologetics the concern is the Cosmological Argument is its simply another 'God of gaps' (@raygrant mentioned). As further evidence is gleaned about what occurred before the Big Bang then the existence of God becomes less certain.
I would simply point to the resurrection and evidence of the resurrection as undisputed evidence of God. No scientific theory could ever account for someone rising from the dead and entering into an immortal body that utterly defies the laws of physics.
Note Astrophysics isn't my subject its genetic analytics - there is some (not huge) cross-over in the analytic methods used with astrophysics. In any case, I can definitely and unequivocally say the resurrection is God alone. There is absolutely no scientific wiggle room here.