I have heard some say that a Muslim cannot make a Christmas greeting to someone or to celebrate in the party of the holiday. These people have caused a debate where a debate should not exist, I think. One can proceed as Jews do by saying: "Happy holidays," which that just crams all the holidays into one greeting. Another way of wishing well to Christian friends would be: "I wish you all the best," or "Like always, I wish you health, happiness and peace." I have met Muslims who participate in Christmas celebrations and then those who stir clear. Also, I have seen Jews who refuse to decorate Christmas tree and Muslims who do it with their Christian friends.
The reason why the preacher said the Christmas greeting might be worse than a murder is NOT because it is Christmas, but because of the possibility of committing the worse sin of all, as per Islam, and which would be recognizing that there is any other deity besides God SWT! Islam recognizes small and great sins. Murder is one of the forbidden great sins. However, God SWT in the Qur'an is promising to not forgive--never ever!-- only one of the great sins: belief that there is more than one deity.
Of course, for any sin to be unforgivable in Islam would be to keep going to the sin after you repented (as per Qur'an). Therefore, people who used to believe in idols prior to accepting belief in One God might be forgiven that sin provided they do not go back to idolatry.
So, what about the Christmas greeting? I think it boils down to the actual belief. Is the purpose of the greeting to express that you mean well to someone or to express your personal belief in something? If someone invites you over for a lunch to share with you on the most happiest days for them, would you be rude and decline? What if that person visited you when you invited them over for your holiday?
