The people who don't believe in evolution are the people who don't know anything about evolution. The scientists who don't believe in evolution aren't scientists and they wouldn't recognize evidence if it leaped out of the water and slapped them in the face.
The biggest problem is the use of the word believe. Evolution is not something to be believed or not. The believing isn’t what makes evolution true or not. What determines whether or not evolution is true is whether there is evidence to support it, and, just as importantly, whether there is evidence to refute it. To date, more than 175 years after Charles Darwin began to formulate his theory, not a single piece of evidence has refuted the theory. Not one.
Further, a scientific theory makes predictions about things yet unknown that must be true in order for the theory to be sound: if this is true, then that must also be true. If that isn't true, then the theory crumbles. Evolution predicted that precursors to the trilobite would be found in pre-Silurian rocks. They were. Evolution predicts that Precambrian fossils would be found. Darwin wrote in 1859 that the total absence of fossils in Precambrian rock was "inexplicable" and that the lack might "be truly urged as a valid argument" against his theory. 94 years later when such fossils were found, it turned out that they had been abundant all along. They were just so small that it took a microscope to see them.
Evolution predicts transitional animals. They must have existed in order to evolution to be valid. For example, there are two kinds of whales: those with teeth, and those that strain microscopic food out of seawater with baleen. A transitional whale must have once existed, which had both teeth and baleen, otherwise evolution cannot be correct. Such fossils have since been found.
Most animals make Vitamin C inside their bodies. Some do not, mainly the simians (monkeys, apes, people), and creatures like guinea pigs, most bats, a few others. Other primates do, like lemurs, bushbabies, lorises. It was predicted that humans are descended from creatures that could do this, and that we had somehow lost this ability. The question evolution asks is why? When human DNA was studied, scientists found a gene which is identical to the Vitamin C gene in dogs and cats and lemurs. However, our copy has been turned off. It doesn't function. Why? It doesn't need to, because our diet is rich in Vitamin C.
The theory of evolution, and it's predictions, have been supported and confirmed time and time again by the very different scientific disciplines of paleontology, archeology, geology, biology, microbiology, anthropology, genetics, chemistry and even agriculture. We see evolution happening every day with rapidly evolving microbes like the flu and HIV viruses, with the embryos of fruit flies and other insects, and with the birth of every new living thing. Evidence for evolution continues to be accumulated and tested.
Creationism, on the other hand, has no evidence, makes no predictions, and refuses to allow the question of "why?" Creationism is the brick wall of knowledge and learning. Creationism makes you stupid. To dismiss actual evidence in favor of a closely held belief is willful ignorance. It is prideful ignorance. Knowledge is based on facts, belief is not. Those who dismiss evolution as "it's only a theory" are stupid. It is stupid. They don't know what a theory is. They think a theory is an opinion of some kind of conjecture. But in science a theory has the much stronger meaning of a "well-substantiated explanation" that can be used as principles of explanation and prediction for something. The theory of relativity, the theory of gravity, the theory of heliocentrism, the theory of thermodynamics, the theory of plate techtonics, the theory of evolution - each and every one of these is "it's only a theory."
