Its actually a pretty common concern among trads: The fear is that by wishing the heretic or atheist "merry Christmas," you are confirming them in their indifference, or committing an act of indifference yourself, or implying their situation is OK by overlooking the religoius issue/nature of the salutation.
I think it comes as an offshoot to the Biblical injunction from 2 John 1:10 - "If any man come to you, and bring not this doctrine, receive him not into the house nor say to him, God speed you."
In other words, if you can't even say "God speed you," much less can you wish him merry Christmas."
The footnote to this passage in the Challoner translation reads:
"But by this is not forbidden civility, kindness, and a sincere charity, for all men, by which we ought to wish and pray for the eternal salvation of every one."
But this is not the same thing as rejoicing with another regarding the celebration of a religious doctrine (i.e., the Incarnation), particularly when the one you salute rejects the Incarnation (or is a member of an heretical sect).
Moreover, the Glossa Ordinaria says:
[10] If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him God speed:
If any man come to you. As John in Ephesus was entering the baths to wash and saw Cherintus, he went out saying, "Let us flee from here, so that the baths themselves in which Cherintus, enemy of the truth, is washing, may not corrupt us." Therefore he did what he taught. For then the apostles and their disciples were using such caution in religious scrupulousness that they wouldn't even tolerate to share a word with one of those who had parted from the truth. (Bede Expo. On 2 John)
Receive him not. Because it looks like a sign of approbation of (his) error, indeed we say this to the friends whose works we like, and if by case you weren't deceived, yet others may be deceived through this friendship of yours, who would believe that you like them, and so would trust them.
https://sites.google.com/site/aquinasstudybible/home/2-john/glossa-ordinaria-on-2-john
And St. Clement of Alexandria says:
"He says, If any come unto you and bring not this doctrine, receive him not into your house, neither bid him God speed; for he that bids him God speed is partaker of his evil deeds. He forbids us to salute such, and to receive them to our hospitality. For this is not harsh in the case of a man of this sort. But he admonishes them neither to confer nor dispute with such as are not able to handle divine things with intelligence, lest through them they be seduced from the doctrine of truth, influenced by plausible reasons. Now, I think that we are not even to pray with such, because in the prayer which is made at home, after rising from prayer, the salutation of joy is also the token of peace."
https://sites.google.com/site/aquinasstudybible/home/2-john/st-clement-of-alexandria-on-2-john
So in light of these passages, many have difficulty understanding how it can be permissible to wish a heretic merry Christmas, but not "God speed you."
Actually, I'm going to have to dig into this a bit myself, because I'm not really sure what the reconcilliation is either.
Okay, well, I had another reply typed out but my log-in expired so I am thinking that is Providential. Sean, thanks for the heavy-duty Philosophical answer, if you will. Yeah, not really what I was expecting or care about. Kind sir, we are not in the same league. :-) That kind of stuff is way over my head.
I was merely trying to figure out if it was some sort of sedevacantist thing, or some type of regional concern of Trads, or what. Honestly, I can't see it being a common concern, but it seems to be in your area or interactions. I find it strange. That's it! I went over the top to give a real life example for the OP to try to relate to, but I see I am obviously not reading what he is after correctly.
I am thinking scrupulosity might be at play here now, so whomever gave advice for the OP to talk with a Priest is probably best. No offense to Cryptonite.
I could care less who gets a Merry Christmas greeting really. If they turn out to be atheists, that is not my problem, it's theirs. ;-) God knows my heart and intentions, and I am not going to stop and analyze each and every person before I consider wishing a Merry Christmas or not! That's God's job, not mine.
Thanks again. Please, also, my apologies for stepping on your toes or anyone else's. I am not here to create problems, but I see that there are some posts I will be ignoring from now on. I am in the Beer League for posts. If you want to debate with hand-rolled cigars and stuffed leather chairs while wearing a bow-tie? I am not in that same caliber and have no interest in pretending to be there.
(So there's a side note for anyone reading this and recently yakking about my having some sort of "lack of humility," LOL.)
Merry Christmas ya filthy animal! ;-)