According to this article in the Harvard Civil Liberties Law Review here:
The ecclesiastical abstention doctrine, also known as the church autonomy doctrine, is a longstanding common law doctrine that guides courts when a case would require the court to decide a religious question. In its most distilled form, the doctrine counsels that if a case would require a civil court to decide a matter of religious doctrine, the court should either refuse to adjudicate or defer to the relevant religious hierarchy. Watson v. Jones, an 1871 Supreme Court case is often cited as the basis for the doctrine in the United States, and grounds the doctrine in the Free Exercise Clause of the First Amendment.
This doctrine was just applied in a legal case in Michigan where the pastor officiating at a funeral made public that the deceased person had committed suicide. The nature of the death was not public at the time, and the parents claimed that the pastor's sermon caused emotional distress and an invasion of privacy. A Michigan state appellate court dismissed the case by claiming that this is a religious matter and not something for the court.
The boundary between "religious" and "not religious" is continually negotiated, and the ecclesiastical abstention doctrine is a key idea in that negotiation that determines the rules that govern different aspects of religious life.