“You don’t “believe” in evolution…” “… you either understand it or you don’t”
This is a silly bumper sticker-like meme. It is not only condescending but also presents equivocal language and a false dilemma. The equivocation comes in the word “evolution”. “Evolution” has different meanings that should be described, however, the nature of a meme is such that it asserts opinions with the goal of gaining attention quickly. Memes do not evaluate issues in depth and are attractive ways to spread ideology without really wrestling with the facts.
For the different meanings of “evolution” see the work Evolutionary Biologist, Francisco Ayala as well as Biochemist Michael Behe. Each of these men agree that the mechanisms of neo-Darwinian evolution (natural selection and random mutation) are inadequate by themselves to fully explain the evolution of living organisms AND that the topic does not have consensus in the scientific community. Most biologists believe in evolution in the sense that creatures can change over time. However, this does not regard the inadequacies of neo-Darwinian mechanisms, which pop culture dogmatically worships. Further, the false dilemma can be seen as the word “understanding” does not equate to “agreeing with”.
Many, including the scientists mentioned above, understand completely the intricacies of neo-Darwinian evolution better than any layman like me or the creator of this meme will ever understand it. However, understanding it and agreeing with are not the same. The meme seems to suppose that if one truly understood it, then they would automatically and emphatically support it, which is not necessarily true. So there is a third option by which one can escape the false dichotomy, namely that one can understand and disagree. These two fallacious attempts to influence people also work together to build a straw man falsely depicting evangelical Christians. The straw man in this case attempts to draw a caricature of those who believe transcendent causes as all holding to irrational beliefs. This is uncharitable and untrue. In my experience, evangelical Christians and other religious people are often more willing to follow the evidence where it may lead than a staunch naturalist who will never let go of her dogmatic beliefs in the lack of anything beyond nature.