As wrong as it is, I wouldn't call "medicine" a medieval ideology. Although I will evoke the ancient medicine (the healing arts indistinguishable from magic) and surgery (the art of cutting other people open) divide. In the ancient world medicine and surgery weren't the same thing and they shouldn't be in this thread either.
However, here's what Quartz wrote about the subject:
Most Americans assume the practice is medically useful, like a vaccination. The medical community agrees: both the U.S. Centers for Disease Control and Prevention (CDC) and the American Academy of Pediatrics (AAP) claim the benefits of circumcision outweigh the risks, citing evidence that circumcision lowers a man’s risk for HIV, urinary-tract infections and penile cancer. But a closer look reveals that, at least in the industrialized world, the health benefits of circumcision may be negligible.
So they may be wrong to do it based on the evidence, but there is at least some evidence. In any case, it's a Jewish conspiracy, which is another ancient ideology.