When making a case for G-d's existence, theists invoke the Design Argument. The essence of the Design Argument is that design implies a designer. The existence of a suit implies a tailor. The existence of a building implies an architect and construction workers. The argument is short and easy to comprehend.
I want to study the merit of the Design Argument itself in a later entry, but I wanted to study a variation of this argument because theists take the argument to a different level in what is called the Fined-Tuned Universe Argument. The basis of the argument is, as renowned theist and scientist Gerald Schroeder states, that "the laws and constants of nature are so 'finely-tuned,' and so many 'coincidences' have occurred to allow for the possibility of life, the universe must have come into existence through intentional planning and intelligence."
The essential argument is that if any one of these constants were different, such as the electromagnetic force or gravity, life on Earth would not exist. Atheists retort this with the Multiple Universes Argument, which states that with all the universes out there, that it is statistically possible to have more than one planet with life on it. The theistic argument of the Fine-Tuned Universe assumes that the variables that caused Earth to come into being are the only possible ones for creating life (e.g., life does not automatically have to be carbon-based). If there is more than one way for a universe come into being, then the statistical improbability of random inception would be underminded. Theists will retort this theory with the fact that it hasn't been proven yet. An atheist can just as easily respond to that by saying that it hasn't been disproven.
The universe seems to be fine-tuned. Although being fine-tuned implies a designer, that does not automatically mean that there was a Designer that created the universe. Just because there could be multiple universes out there does not disprove G-d's existence. Why are none of these arguments proof or disproof of G-d's existence? Because none of the arguments address primum movens, which is just a fancy way of asking the question of how the universe began. I'll save that one for a future date when discussing the Designer Argument itself, but would like to end with this.
The precise conditions under which the universe was created are amazing. If anything were slightly off, we would not be here. Although we have not proven the existence of extraterrestrial life, that does not mean that it does not exist. Some other planet may exist with different conditions that brought about life. Regardless, the fine tuning of the universe makes me much more prone towards theism than atheism. This can be convincing argument when considering everything else, but unto itself, it is not a proof.