To elaborate on what the No True Christian fallacy is, it is a tongue in cheek use of the No True Scotsman Fallacy. I feel that the reasosn many atheists used the tongue in cheek name is probably bacause theis fallacy gets used so often by Christians that it in the future it may no longer be called the No True Scotsman
Now the no true Christian fallacy is dumb beyond dumb. As we all know the Bible is the great big book of multiple choice. You can literally justify anything using the Bible. Even so much so that I sometimes now debate ridiculous claims that "No True Christian can (insert ridiculous claim here)" using biblical evidence. Granted I usually lose, after all theists do not appreciate an atheist using the Bible to justify anything, but it gets the point across.
For example, when a Christian says the Nazi Party was not Christian then they are using the no true Christian fallacy as it is difficult to get around the following proclamation at a Nazi Party Rally by Adolf Hitler.
"We demand liberty for all religious denominations in the State, so far as they are not a danger to it and do not militate against the morality and moral sense of the German race. The Party, as such, stands for positive Christianity, but does not bind itself in the matter of creed to any particular confession. It combats the Jewish-materialist spirit within and without us, and is convinced that our nation can achieve permanent health from within only on the principle: the common interest before self-interest. "
In fact usually the only way that Christians try get around this is to point out Hitler actually had anti-Christian views. However, these are dubious claims in the first place as the book (Hitler's Table Talk, 1941-1944: His Private Conversations) where these views come from has been academically debunked as not Hitlers views by writers such as Ian Kershaw in his book on Hitler and other scholars. For a broader understanding you can also look here.
Then again, if the Christian still disagrees you can point him towards this picture, where the Nazi belt buckle says "God with Us".
Surely they will then tell you that is because America is a Christian nation.
This is why the No True Christian Fallacy and the Christians that use it have some serious intellectual dishonesty.