The two most common arguments for the existence of God today are based on the beginning of the universe, discovered in the early 20th century, and the fine-tuning of the universe, much more recently discovered by scientists in the last half century.
The fine-tuning of the universe refers to the fact that the universe is constructed by, in effect, various sets of constants that, were they to be any different by a miniscule scale, life in its entirety not only would not exist, but could not exist. As Martin Rees reveals in his book Just Six Numbers, explained by Rich Deem;
Another finely tuned constant is the strong nuclear force (the force that holds atoms together). The Sun “burns” by fusing hydrogen (and higher elements) together. When the two hydrogen atoms fuse, 0.7% of the mass of the hydrogen is converted into energy. If the amount of matter converted were slightly smaller—0.6% instead of 0.7%— a proton could not bond to a neutron, and the universe would consist only of hydrogen. With no heavy elements, there would be no rocky planets and no life. If the amount of matter converted were slightly larger—0.8%, fusion would happen so readily and rapidly that no hydrogen would have survived from the Big Bang. Again, there would be no solar systems and no life. The number must lie exactly between 0.6% and 0.8%.
So, the universe is finely tuned for the existence of life. But how did the fine-tuning come about? Since of course it is far too improbable for the constants to turn out the way they are by chance (the probability that the cosmological constant would be set just rightly to allow life is something like 1 in 10^60), chance is obviously out of question. Obviously, the most intuitive option is that God designed the universe. Indeed, it seems to perfectly correlate with what we are symbolically told in the opening chapters of Genesis, that God created all that exists to our aid so that we may rejoice in it and worship God its Creator (The heavens declare the glory of God, and the expanse proclaims the work of his hands -Psalm 19:1).
An atheist of course, cannot accept that. So, we are given an alternative way to understanding the constants — “Maybe there’s a multiverse,” we are told, “Maybe there is an inexpressible number of universes so that at least one of them had to have the constants we see in our universe.” So, the question arises, is there a multiverse so that we can explain why the constants of the universe are so finely tuned as to allow the existence of life (and moreso, advanced biological life)? The answer is, probably not. There seems to be no evidence for this supposed multiverse, even though we can look some 90 billion light years in diameter in our universe. Secondly, as the great philosopher Richard Swinburne puts it, “To postulate a trillion trillion other universes, rather than one God, in order to explain the orderliness of our universe seems the height of irrationality.” The multiversal hypothesis, when putting it alongside the competing God hypothesis, violates Occam’s Razor. According to Occam’s Razor, when understanding a phenomenon or something of some sort, we always ought to prefer the explanation that makes the least amount of assumptions.
The multiversal hypothesis seems to make an extraodinary, if not literally infinite number of assumptions (as some variations of the theorem postulate an infinite number of universes). Firstly, you must not only assume your numberless universes exist, but you also need to presume the existence of a mechanism that exists beyond the confines of space-time in order to continually produce these universes, there must be a universe producer of some sort (you can’t use God), in which no evidence suggests that such a thing is even naturally possible. This is irrational. God is just one assumption, and as some philosophers believe, God is an infinitely simple assumption (since complexity is determined by the parts or constituents that make up the whole, and God is not made up of any parts or constituents).
And, to me, the most daunting problem with the multiverse is that it explains absolutely nothing — it just pushes the cause one step back (contrast that with God, who is the uncaused first cause), which would require yet another explanation on top of itself to explain the multiverse! The multiversal hypothesis is shredded by Occam’s Razor and explains nothing. Therefore, the theist can once more rejoice in the science and philosophy that has affirmed his belief in God.