I would agree that possibly time didn't exist before the Big Bang, for time seems to be a dimension that needs various referents to be legitimated, which may or may not have existed before the Big Bang. To me (as well as to many, I presume), a legitimate question is "What was it that can be described as 'existing' in that infinitesimally small point before the Big Bang?" What was that, what was its nature, and what was the causative mechanism that stimulated the Bang? If it was truly nothing, then the universe had a beginning, which means we are philosophically justified to speculate a first cause. If it was truly something, then something is eternal, and we are philosophically justified to conclude that the first cause didn't need a causative mechanism because it is self-existent. Either way, we can have a good discussion.

I know Christians often say that something always had to exist—either God, matter, energy, or even the infinitesimally small point before the Big Bang. But we don't know that at all. We don't even know that time existed before the big bang, in fact, a common theory in the scientific community is that it didn't but true to the nature of science, we haven't stopped looking (well, except people that know they have the answer already, even if they are wrong).

Or we just cop out and say "the bible says it, so that's the conclusion".