Graduates of medical schools used to be required to take the Hippocratic Oath, which began: "I swear by Apollo Physician and Asclepius and Hygieia and Panaceia and all the gods and goddesses, making them my witnesses, that I will fulfill according to my ability and judgment this oath and this covenant:..."

So, the origin of the oath communicated a religious ideal, though a pagan one. It's purpose was to bind a doctor to a standard higher than himself.

In modern times, the Oath has been optional, but the sense of a moral, even religious standard, that would control the ethical decisions a doctor must make has remained. Most doctors respect the religious beliefs of their patients, except when they might conflict with sound medicine and the best interests of the patient. In circumstances involving, for example, Jehovah's Witnesses -- who mostly oppose blood transfusions, or Christian Scientists, who mostly eschew medical treatment -- patients need to make their wishes known in writing before treatment, such as in a will or other legal document that spells out precisely what an individual wants and does not want done.

In the case of children, doctors and the state are usually able to override parental wishes that are based on religion and not medicine if the best interest of the child demands it.