Darwin gave the answer to the apparent illusion of the irreducible complexities of life. From Darwin (and friends) we now know that the original self-replicating molecule/molecules gave rise to all the different complexities of life. Humankind was knocked down off of its high pedestal of self importance and ignorance and brought down to the level of all other living organisms. No longer were we justified to say that we were created in a Gods image, we were just like all of the other animals utterly unimportant in terms of the universe as a whole. And as we look up into the sky with our modern day telescopes we find that we are on a single planet in a sea of galaxies each containing estimates of ~100 billion stars, each with their various planets and moons and most likely life of their own.
I argue that Dawkins was completely justified in making the statement that "Darwinism allowed one to be an intellectually fulfilled atheist." But, I would go one step further and say that every single scientific discovery fills a hole that was once left to the dead Gods of old. Darwins contribution while not sufficient to get rid of all of the different beliefs in a God, sure did well to justify the disbelief in the Judeo-Christian God.
"From so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."
the concluding passage from, The Origin of Species, Charles Darwin 1859.