Something well-known by writers, lawyers, politicians, and public speakers (including preachers) is that often the connotative meaning of words carries more value than the literal. By including "civil war" when the preacher could just as easily left it out or used another adjective, he is invoking all the gravity of the Civil War, and especially evocative is the fact that it has been suggested before that militant Trump supporters may take violent action should he be removed.
In the context of being impeached, this quote can easily be considered threatening.