To call someone a heretic is to bring a very serious charge against them. The word is often thrown around amongst Christian groups seemingly without an understanding as to what they are actually saying about someone when they call them a “heretic.”
The phrase, “The spirit is willing, but the flesh is weak” is, I think commonly understood to mean that someone wants to do something but is unable to due to physical limitations. However, I think there is more to the phrase than denoting mere physical inability.
The Bible is the most important book in Christianity and perhaps also the most misunderstood. It has had some of its passages ripped out of context and used as justification for all sorts of abuses and atrocities. Slavery, racism, misogyny, various political causes, etc. have been attempted to be justified on the basis of various Bible passages. In fact, I’m willing to bet that with enough creativity you can find a Biblical justification for almost anything that you want to do. So there is a lot of clarification that needs to be done in regards to what Christians believe about the Bible, even among Christians.