Argumentum ad populum is when the popular opinion is said to be the correct one just because it is popular. It's an interesting advertising strategy, but when you're more interested in selling a worldview rather than a product, it's useless. After all, at no point in history has truth been a democracy, and at many times in history, treating it as if it was has lead to some serious errors, such as the widespread acceptance of the geocentric model.
Where Christianity is concerned, argumentum ad populum becomes a double fallacy. It adds an element of the straw man argument. The straw man argument is when someone misrepresents a particular worldview or argument and proceeds to attack the misrepresentation, rather than the view/argument itself. Using argumentum ad populum against Christianity is not a true straw man, but it does have an element of it in that Christianity was never expected to be the majority opinion.
One particular example, and probably not one most people would expect me to use in this particular article, is the Catholic argument that Catholicism is still the largest denomination in Christianity. But of course, if many people find the path to destruction, but few find the path to eternal life, would we expect many to find the correct denomination? Of course not! It would make more sense to look for the smallest denomination and say that because few find it, therefore they are correct. Of course, this would also be erroneous, but it at least makes sense in light of Matthew 7:13-14.
When it comes to heathens vs. Christians, a far more logical way to dispute is to look at the evidence. When it comes to Christians vs. heretics, a far more logical way to dispute is to look at what the word says. In both scenarios, arguing from majority opinion will always be a fallacy, no matter what it's used against, and a double fallacy when used against Christianity.