When we say " Merry Christmas" to our Christian friends, we certify the resurrection of Christ, means son of God.
How big a SIN it is, lets read.
19:88 As it is, some assert, “The Beneficent has betaken a son!”
19:89 Indeed, you have put forth something monstrous!
19:90 Whereby the heavens are almost torn, and the earth is split asunder, and the mountains fall in ruins!
19:91 That men should ascribe a son to the Most Gracious.
19:92 It is not befitting for the Majesty of the Beneficent that He should choose a son.
( if a mountain can hear that, it would have broken itself into pieces. )