Who is Christian and who is not, is a question that has been around for centuries. The first major problem with who was and who was not a Christian started early on in the faith. We have many tid bits recorded by the Apostle Paul addressing the subject of true faith. Latter on in history Christian faith was sometimes defined by where you lived and what group was in charge of the state church.
Sometimes the control of the state overshadowed issues of faith for the individual. Other times individuals were successful in addressing and correcting issues within the frame work of the state church. Throughout this period of history there was a divide that most people recognized. The division was one of faith/action and state acceptance. A person could be accepted or rejected by "the church" based on conforming to the official doctrine.
If an individual "sinned" they could be kicked out of the church. In some cases this involved serious social consequences. A person who had been "read off" at mass might suddenly find his line of credit suspended at the bakers. A person charged with engaging in some form of official heresy might loss trading privileges or access to local markets.
When Henry VIII King of England decided to dump his wife and the church refused to let him, he decided to dump the church. This caused Englishmen of conscience more than a little anxiety as they had to chose a side, and they knew full and well following their faith might cause their downfall.
In the US today things aren't that tense. People can pick and choose a church with about as much thought as they select which fast food drive thru to visit afterwards. If you don't like Church A, Church B is right around the corner. People church hop all the time.
What do you do when a Church decides that things that scripture declares an "abomination" that will cause the unrepentant participants to go to hell are now "Christian" and "Holy"? Of course you can leave and hop over to another church with teaching more to your liking. What do you call the group you just left? Are they still "Christian"?
Christian denominations haven't always gotten along and accepted each other. There has always been a bit of distance. Some of that is preference based and some is theological based. While we haven't always agreed to worship together we have generally agreed who is and who is not a Christian. Thus Jehovah's Whiteness, Mormons, Seventh Day Adventists, and despite the name, Christian Scientists have been regarded as "not Christian" while most of the rest, despite large differences in doctrine are.
Now that the Presbyterian Church has embraced the example of the Episcopalians they will slide further into irrelevance. The question becomes should we officially recognize that they are no longer following the Bible?
FWIW I blame Calvinism, Biblical relativism and Feminism, in that order, for the downfall of the Presbyterian Church. The truly sad thing is that this course of action is going to serve to harm the cause of Christ, despite the professed belief that "acceptance" and "tolerance" are somehow "Christian virtues".