I often hear evolutionists claim that evolution is not a theory, but a fact. I argue that this is not so. Organisms living or fossilized are facts. The characteristics they share, such as backbones or hair or flowers, are facts. (And sometimes we get confused about these things, too.) Evolutionary theories, one or more of them, have been constructed to explain the existence of these biological facts of planet Earth. To call evolution a fact makes it hard to criticize, to re-examine, to amend. This makes evolution more of a focus of faith than an argument of science.
To call something a theory doesn’t make it weak, as some may think. On the contrary, theories are strong because they must stand up to testing. A theory that manages to remain a rational explanation in the face of empirical evidence or criticism is stronger than something you either believe or don’t.
I confess to being the suspicious type. When I hear evolutionary biologists claim that, “Evolution is more than a theory. It’s a fact.” they are almost always talking about neo-Darwinian evolution, with natural selection and survival of the fittest and all those other claims that are rather nonsensical in the face of biology. This elevates a rather poor excuse for a theory to a revealed truth. This makes for bad science! Like Karl Popper said, “All knowledge is human.” The idea that everything is open to criticism is the thing that makes science strong.