It’s a bit odd how religious persons say things like “Science is for understanding the natural world, faith is for understanding the supernatural or spiritual world” as if these two methods of learning what is real are equally valid. They clearly are not.
“Science” basically means “testing.” You formulate a theory, devise a way to test it, and judge the results to see what’s true about the natural world. Now, it is true that some theories can’t be tested or haven’t been tested and are inappropriately presented as fact or likely to be fact. It’s also true that sometimes science is wrong — tests are flawed, good tests yield inaccurate results due to unexpected phenomena, or results are misjudged or misinterpreted. Yet over time, science grows more accurate. Tests are repeated over years, decades, and centuries, giving us further confidence in findings. New individuals administer such tests, weeding out biases. New tests are designed, looking at long-studied phenomena from different angles and in new ways. In these ways, 1) the ability to actually test ideas and 2) improved understanding over time, science helps us know what’s true.
Faith has neither of these things. First, it might be noted that when you hear the statement in the first paragraph, the speaker is typically talking about one faith, his or her own. Christian faith helps you understand the spiritual world, the true spiritual world. Hinduism, Scientology, Islam, and Buddhism won’t help you know the supernatural world, those are of course all false religions. In any case, we’ll assume a more open-minded stance, because some do believe that there is truth in all faiths, that all roads lead to Rome.
Where science is an ever-growing body of knowledge based on testing over time and into the modern age, faiths typically present more or less fixed bodies of ideas based on writings from comparatively primitive ancient cultures — desert tribes from the Middle East, for example, in the case of Islam, Christianity, and Judaism. Such writings describe higher powers, afterlives, the meaning of life, and so on. Ideas, supposed knowledge, of the supernatural world. Unfortunately, there is no way to test to see if any of these notions are true. The ideas could easily be man-made fictions. You may believe your Hindu or Christian or Islamic faith helps you know and understand the spiritual world, but, to put it bluntly, that spiritual world may not exist. There is no test to run to find out. (And no, Jesus being resurrected is not a “test,” nor are miracles, answered prayers, or feeling God speaking to you. The obvious problem with this poor counterargument is that these things cannot be tested for validity either. They could easily be human fictions and imaginings as well, as explained in detail elsewhere. Think about it. If someone doubts photosynthesis, you can teach him how to test to see if photosynthesis is real; there is no test you can use to show him a god or goddess is actually speaking to you, that it isn’t just in your head. “Try faith yourself, you’ll see the proof, believing is seeing” is the best a believer can say, possibly just drawing the fellow into human fictions and imaginings as well — there is no way to test to know otherwise.) This is in stark contrast to science; we can have confidence that the natural world exists, and we are able to put ideas to the test to actually see what’s true or false, or most likely to be so.
Linked with the lack of “knowledge” verification or falsifiability, of course, is simply the fact that ideas about the spiritual world cannot grow more accurate over time. Ancient scriptures aren’t typically added to. (In modern times, at least.) Texts are of course reinterpreted, gods are reimagined, ways the faithful think they should live change. For most American Christians for a long time, God was fine with the enslavement of blacks and the bible was used to justify it, without difficulty given what’s in it. Today things are quite different. Religions may change as societies do, and the faithful may feel they gain more knowledge by studying the scriptures more deeply, but no one will ever discover that we only spend 1,000 years in heaven, not eternity. Christianity won’t change when someone announces, after much research, that God has a couple wives up in heaven. Newly discovered ancient writings won’t become holy scripture. As stated, it’s all a fairly fixed set of ideas about the supernatural realm. The immutable nature of religious “knowledge” is of course celebrated by the faithful — everything we need to know about the spiritual world was written thousands of years ago, we don’t need more than what God gave us or any improved accuracy, everything’s accurate and must be preserved. (This is in contrast to scientists, who can make history, really make names for themselves, by disproving long-held scientific theories; there’s a personal incentive not to preserve doctrine but to blow it up.) But the supposed knowledge and its assumed accuracy are untestable and could easily be false, and there’s no process of gaining more knowledge or improving accuracy over time to really hammer out if these things are false or true. Imagine if science was like this — no way to know if germ theory is correct, no centuries spent gaining more information and developing better and better medicines. In its ability to test ideas and improve understanding as time goes on, science, unlike faith, is an actual method of learning what is real. (All this should not be surprising, given that “faith” is often defined, by believers and nonbelievers alike, as “belief without proof.”)
In sum, science is a useful tool for gaining objective knowledge about the natural world. Faith is simply hoped to be a tool for gaining what could easily be imagined knowledge about an imagined spiritual realm. These things are hardly the same.