The probability only became quantifiable once a separate problem was solved; allowing scientists to draw up a new question using a better understanding of the old question.

until we can account for all of the variables surrounding the creation of our Universe and the events that took place upon the creation of our Universe the probability can never be anything more or less than 50/50.

For as long as there is a single "wild card" the probability will be un-quantifiable and will revert to basic odds that the existence of God will be either true or false.

Hypothetically, even if you have a probability of 99.99999% that God does not exist and yet you also have a single variable with an unknown magnitude then you still can't answer the question certainly as the entire equation is still off-balance.

The probability that I personally assign to God existing in some manner or another is much greater than pink Unicorns existing. Still, un-quantifiable...