At the end of every chain of reasoning there is a big chasm, a gaping pit of uncertainty. This is usually not a problem, but it really depends on how far you want to go into the rabbit hole of what we know and what we can know. I know that I exist, and I assume that you exist, but that's really just an assumption. And, of course, I could be a brain in a jar being fed all of these thoughts and sensations and memories. But barring that, there really is a lot that scientists know.
Here's the thing. We are curious. And for centuries we have been looking for explanations, reasons why the sun goes away at night, why do plants grow, how do our bodies heal themselves and so on. Thousands of years ago, people realized when they did things, they caused other things. So they invented sun gods to control the sun. Since no one believes in Helios or Ra or Quetzacotal any more than the Easter Bunny, then we would assume it to be a human invention as a kind of primitive science. The development of a sun god did not increase the knowledge about the sun or the solar system, but it gave humans an understanding of the world. It was a wrong understanding, of course, and as science and technology and knowledge developed, our understanding actually grew. The universe didn't change, of course, and the sun's behavior is still the same as it was thousands of years ago, but instead of assuming that someone pulled it across the sky in a chariot, we know how the whole thing works. And the entire history of scientific development is marked with wrong understandings; the birth of new ideas are specifically the result of the death of old ones. So in that sense also, we have to take it on what you might call faith. If we lived six hundred years ago, you would have been forgiven for thinking the world was flat, you might even have been burned at the stake for suggesting otherwise. But now we know better. Who knows what will be thought about the state of science in a hundred years? The concept of faith simply does not apply to science because that isn't the point of science. Science asks, 'how does something work?' and then seeks the best answer available. Then - maybe ten years later, maybe the next day - someone has a new idea. Maybe the new idea is a better explanation. But this is not faith. This is simply understanding how science works.
Natural Selection is as established a scientific principle as any other. There are many questions that are unanswered, but the basic structure is sound and has stood up to each and every challenge. As far as the gaps in knowledge, a simple 'I don't know but we're working on it' works just fine. So if you wish to label these gaps in knowledge 'faith,' then that is fine. But please understand that I do not. This is what I call faith: A worldview that presumes a god. Two or three centuries ago this may have been fine, but these days there is no need for a god. We don't need a god for the sun, we don't need a god for the seas, and we don't need a god for life either. We know (and by 'we' I mean humanity in general - I certainly know very little, although I probably know a good deal more than most fully educated scientists of the 1700s) how life could have come about - true, we don't know the actual path that life took in many places, but the fact that working models exist means that god is not necessary to make the process happen. This was not an excuse to push god out of the laboratory, but it just turned out that there was no reason for him to be there. And all of this scientific evidence was one of the many factors that led to my eventual loss of faith. If god were so powerful and so intent on making his presence known and understood, then why did he go to all the trouble of wiping away his fingerprints when he was finished? Why did he work so hard to make a world that looked as though he were never there at all? Some say that he put this distracting 'evidence' down to test our faith, but if that was the case, then isn't that just cruel? Why would he punish men for using their heads?
One major missing piece of the cosmic puzzle is, obviously, the beginning of the universe. Christians (and Muslims and Hindus and virtually every other faith) propose that it was god that started the universe. So, since there is no way to know otherwise, I have no real problem with that theory, except that I would point out that labelling the First Cause 'god' says nothing about this god's other characteristics. People who use the first cause as their keystone argument for the existence of god often rely on a list of assumptions that follow: God was the first cause of the universe. (Granted - something must have started everything off.) God then created life. God then created man. God then expects things of man and will judge them. Presuming god as first cause really does not logically allow the argument to proceed any further. And science has no business in presuming god because that does not go anywhere. It stops the process entirely, in fact. And, to take a petty example from the Bible, the Tower of Babel is a grim reminder of how god feels about science trying to understand him.
So here is a benefit to living life without faith. It truly increases my appreciation for the world. It is vast and terrifying and uncaring and nearly unknowable, but through determination and cleverness and massive amounts of luck, we have not only been granted life, but also a chance to understand many of its workings. To say that god created it seems like a big thing to say, but by the time you grant him the ability and desire to do all of this then there isn't much to be in awe over. He's god. Of course he can do all of that. I wouldn't feel the same about touching a billion-year-old meteorite if it was just god that did it. But even if it feels better to have no faith, to have no unfounded, irrational belief in a god, it wouldn't make it true. And here is where we begin to agree again. Here is where the asymptote begins to approach zero, where the idea of human knowledge approaches the line of absolute certainty and we know that we will never truly achieve it. It is entirely possible that we are just pools of consciousness in the mind of a god and all of this is an illusion. But it doesn't matter. I can live my life just fine without any worry about falling off the edge of absolute certainty, and I don't need faith to do it.