Hi Renee, thanks for your response. So, I think we are lacking some clarity here. Let me try and articulate my thoughts more precisely.
First, with regard to someone who claims to be Christian, yet embraces a same-sex, monogamous, permanent relationship, I would not say that they are heretical, or non-Christian. I would say instead that they are accepting a lifestyle that is not sanctioned biblically, i.e. they are being unbiblical. But, being unbiblical is not yet being a non-Christian, since we all are failing to live up to some standard that the Bible holds us to, or failing to understand the Bible properly with regard to what the standards are. So, same-sex couples or advocates of same-sex relationships are not heretical, but their beliefs or their behavior is unbiblical. Two different things.
That said, I reserve the term heretical for people who either fail to believe a necessary truth claim that makes the content of Christianity Christian (e.g. Jesus Christ is God), or who outright reject a necessary truth claim that makes the content of Christianity, Christian (e.g. Jesus Christ doesn’t save us from sin and death). Failing to hold a necessary belief like these or rejecting a necessary belief like these would, I think, make someone something other than a historical, orthodox, Christian, even if they acted religiously or used the term Christian to describe themselves.
I mean if someone says they are Christian but doesn’t actually believe God exists, should I consider them Christian? I don’t see why I should. By the way, there are many people like this. The Catholic philosopher Robert George has recently pointed out that he thinks many of the priests and bishops involved in the most recent abuse scandal in Pennsylvania, are openly atheist in their beliefs (well, not that openly, but they are basically non-believers acting as priests.)
With regards then to determining what the necessary beliefs are, I think we can say with some degree of confidence that all Roman Catholics, Evangelical Protestants, and Eastern Orthodox hold, minimally, to the first three creeds of the Church (Apostles’ Creed, Athanasian Creed, and Nicean Creed), and the first four ecumenical councils (up through Chalcedon in 451). Thus, a failure to accept or a rejection of the propositional beliefs outlined, for example, in the Nicean Creed, would make someone a non-Christian.
With regard to those who embrace alternative sexual ethics, i.e. who are being expressly unbiblical in their beliefs about sexuality and in their own behavior, I would argue that in doing so they are moving away from the aforementioned historic, orthodox Christianity, and there is a greater likelihood that they will come to reject or fail to hold some of the necessary beliefs that denote what historic and orthodox Christianity is. So, I think they are playing with fire, and they should, like anyone else who is going against biblical moral teachings, be encouraged to believe differently about the matter; admonished to stop sinning; and advised to seek counseling so that they can better resist temptation.
Does this make more sense? I hope I am being clearer.