I have, on more than one occasion, offered the phrase "genuine Christians" in my writings. In case the implication is unclear, what I have meant by prefacing the noun "Christians" with the adjective "genuine" is that there are folks that call themselves "Christians" who are, in fact, not genuine. They are lying or they are deceived, but they are not genuine.
Is this a fair thing to say? I would argue that it isn't merely fair; it's biblical. We are told repeatedly in the New Testament that false teachers would be the bain of the Church. They would "come out from us" (1 John 2:18-19). John calls them "antichrists". They would cry, "Lord, Lord, did we not prophesy in Your name?" (Matt 7:22). Tares among wheat. Judaizers. Heretics. They are not a matter of question. They are a biblical certainty. The Bible is abundantly clear that there will be false "believers" in the Church. For me, then, to suggest the same is not revolutionary, but simply submitting to the Word of God.
Last month Peter Heck wrote a piece arguing that it's a sin for Christians to vote for Obama. Understand that he did so in response to a Roman Catholic bishop who said the same thing. When the candidate supports killing babies in and even out of the womb, when he discards what Jesus defined as "marriage" and considers Jesus's view as discriminatory, when he embraces as moral what the Bible clearly calls sin, this writer considers it unchristian to vote for that candidate. He considers the term "Christian Democrat", considering today's Democratic Party platform, an oxymoron.
Is this a fair thing to say? Again, on the surface, given the Scriptures and the facts, it is a general truth. There are fake Christians -- people who perhaps believe themselves to be genuine -- whose choices and words (1 John 2:19) demonstrate that they are fake. It is fair, biblical, and rational. But let me say this. I do not aim to be the one to point fingers at individuals. It is one thing to say "There are false Christians" and it is entirely another to say, "and you are one." Using Heck's example, it is entirely true that the Democratic Party holds large swaths of anti-Christian values. That's a general fact. But arguing that everyone who votes for the party holds the equivalent values is questionable. The same is true in the Roman Catholic Church. Roman Catholic theology has some serious problems. It equates to "saved by works". It violates the concept of grace apart from merit (and does so explicitly). It eliminates imputed righteousness. If you hold to pure Roman Catholic theology, you do not hold to Christian theology. But I can say with absolute certainty that individuals in the Roman Catholic church do not subscribe (or even know of) that theology even though they're in that church. As a generality, then, the theology is wrong, but in terms of real individuals, I would not be able to say blindly, "All Roman Catholics are false Christians." It just isn't the case.
And, look, this isn't limited to "genuine Christians". I would, for instance, continue to stand on the certainty that homosexual behavior is a sin. On the other hand, beating my neighbor who is practicing homosexual behavior over the head with his sin would be pointless. Hunting down those in such behavior to shake my finger in their faces and warn them of God's wrath for that act wouldn't be of value. There are better approaches and, frankly, bigger concerns. (Trust me. No one is going to Hell because they committed a homosexual act. Rather, committing such acts is the product of a sinful life that needs Christ.) So declaring a general truth is not the equivalent to beating an individual over the head with the particulars.
Is it right to call out other Christians or to differentiate between "genuine" and "fake" Christians? Since Jesus did it, I'd have to say it is likely a good thing to do. Am I likely, then, to walk around pointing to people and telling them, "You know, you're not a genuine Christian"? No, not likely. I frankly don't know. So I will call out "genuine" versus "fake" in general, point to the differences, suggest that people in "this" category might need to check themselves, that sort of thing. Since God has not made me privy to the heart, so I cannot know. But for those who would argue that we should keep quiet about such things entirely, I would suggest that you'll need to take that up with Christ ... and the authors of Scripture ... first. You can give me your arguments after you straighten them out.