There are two levels at which these kinds of judgmental attitudes are very problematic. The first and more important one is that it encourages a lack of love and respect between people in their disagreements. The second one is that there is a trend in conservative US politics to try to define the United States as being a Christian nation in which the theology and social practices of Christianity are supposed to be given social and legal precedence over the desires of people who disagree with them. It sounds all nice and good to say that America is a "Christian" nation because it invokes ideas of Christian virtue and love, and this is perhaps what many who repeat this rhetoric mean by it. However, it becomes clear that for many this is an attempt to control the government for the benefit of specific branches of Christianity to the detriment of atheists, agnostics, actual non Christian religions, and any Christian religion that is out of favor with the group trying to achieve power.
To explain why I see the "Christian or not" debate as funny, I need to explain that the Episcopalian church is another one of the "liberal" Christian churches that is accused of being not a true Christian church. If you google search the term "Episcopalian not Christian" you'll find many search results claiming that it is a church straight from hell, a church led by the devil, satanic, not a "Jesus" church, or pagan. Mormonism reacts to this kind of vitriol by doing big advertising campaigns and by giving talks in General Conference loudly proclaiming their belief in Jesus. It almost seems like a kind of a mental inferiority complex for some Mormons who want the whole world and especially the conservative Christian world to acknowledge Mormonism's Christianity. As a result, Mormon's talk about their Christian identity a lot. However, the Episcopal Church hardly seems to react to the noise at all. That is what is so funny to me. Perhaps Episcopalians have self-confidence rooted in their ancient yet new worship structure. I've never heard an Episcopalian talk as if they had anything to prove on the subject.
Probably what is most offensive to me regarding the whole argument is that Christ didn't say that his followers would be known as those who obeyed, belonged to a particular organization, or accepted a specific creed. In fact, Jesus was tolerant of those inspired by him but who weren't technically affiliated with him. Mark 9:38-39 reads, "John said to Him, "Teacher, we saw someone casting out demons in your name, and we tried to prevent him because he was not following us." But Jesus said, "Do not hinder him, for there is no one who will perform a miracle in My name, and be able soon afterward to speak evil of Me. "For he who is not against us is for us."
Jesus gives one criteria for being known as a disciple in John 13:35. "A new commandment I give unto you, that ye love one another; as I have loved you, that ye also love one another. By this shall all men know that ye are my disciples, if ye have love one to another."
Anyone who is inspired by the love of Christ to try to love as Christ did and taught deserves to be known as a Christian. Credentials, memberships, and belief systems help us as humans to organize and apply what we think Christian life is supposed to be all about in very important ways. Nevertheless, it doesn't define what it means to be a disciple of Christ. Love alone does that.