Religion teaches certain standards. One who embraces a belief agrees to uphold that standard and is faithful to it. But there are those who do not live up to the standard. The differentiation is necessary so that bad behaviour by a religionist is not unjustly generalised to include all religionists. For example one terrorist attack does not mean Islam is a violent religion or all Muslims are terrorists. Without justice being applied millions of innocent people stand falsely accused and prejudiced against due to over generalisations.
A true Christian does exist just as an unfaithful one does and both should be judged on their individual merits not some sweeping generalisation.
For what it's worth, I've read through the thread and carefully considered the arguments going on here. I actually agree with you on this one.
I think when people are accusing you of making a No True Scotsman Fallacy, they're making a sort of category error. When you use the word "Christian" you're referring to someone who tries to follow the example that you believe Christ set and is living in line with his teachings, right? So clearly someone who is not making a concerted effort to live in that way would not be a true Christian.
You aren't saying that they aren't Christian. You said they are Christian in name only, as in, you are aware that they fit into the broad category of being Christian in the sense that they might share a variety of doctrinal beliefs or go to Church or what have you, but they do not fit the narrow understanding of what a Christian is as defined within the context of your religion.
That's not a No True Scotsman. The issue with the No True Scotsman argument is that "Scotsman" is literally defined as a citizen of Scotland, and so it does not
imply any sort of ideological or behavioral criteria. "Christian" does
imply that criteria, at least within the framework of your religious worldview. (ETA: And, I would argue, both etymologically and historically!)
At the same time, saying that someone is not a "true Christian" is not always comforting to a non-believer, because it makes it look like you're just as quick to condemn others as the people you label "unfaithful" Christians. I speculate that users might be bringing up the No True Scotsman Fallacy because they feel some sort of hypocrisy in what you're saying, but they're just articulating that intuition with the wrong language. Added to that, I think there is a real sense in which people in this thread do not care about your own interpretation of Christianity at all, because it's just one of many to them; thus, from their perspective, the division between a "true Christian" and an "unfaithful Christian" is idiosyncratic and arbitrary. (ETA2: Partially because they don't believe in Christianity, so there is no such thing as a "true Christian" in that sense to them)
It's not a No True Scotsman, technically. You aren't committing a logical fallacy just because people disagree with you about what constitutes a true Christian. I think you're right to defend your use of the term here and you're explaining it adequately.