Allah says strong words against it "Indeed ye have put forth a thing most monstrous! At it the skies are ready to burst, the earth to split asunder, and the mountains to fall down in utter ruin, That they should invoke a son for (God) Most Gracious. For it is not consonant with the majesty of (God) Most Gracious that He should beget a son." Surah Mary Ch.19 V.88-92
If we still engage in this "Merry Christmas" it can make us fall into Kufr or Shirk. Our duty is just to inform people about this big sin and remember giving Hidayah (guidance) is in Allah's hands. Please avoid it and save yourself from doing this very big sin.