From Islam QA:
It is not permissible to congratulate the kuffaar on their festivals in any way whatsoever
What is the ruling on eating the food (rice, meat, chicken or cake) that is given to us by a Christian friend that he made for his birthday or for Christmas or the Christian New Year? What is your opinion on congratulating him by saying, “Insha Allaah you will continue to do well this year” so as to avoid saying Kull ‘aam wa antum bi khayr (approx. “season’s greetings”) or “Happy New Year” etc?.
Praise be to Allaah.It is not permissible for a Muslim to eat things that the Jews and Christians make on their festivals, or what they give him as a gift on their festivals, because that is cooperating with them and joining in with them in this evil, as is explained in question no. 12666.
It is not permissible for him to congratulate them on their festivals in any way whatsoever, because that implies approval of their festival and not denouncing them, and helping them to manifest their symbols and propagate their innovation, and sharing their happiness during their festivals, which are innovated festivals that are connected to false beliefs that are not approved of in Islam. See also question no. 47322.
And Allaah knows best.
“‘Wishing “Merry #Christmas” to #Christians is worst evil’, scholar tells Muslims,” by Inemesit Udodiong, Pulse, December 26, 2016:
Dr. Zakir Abdul Karim Naik has revealed an interesting theory on Christmas and Muslims, who offer season greetings to Christians.
The Indian Islamic preacher says that wishing ‘Merry Christmas’ to Christians is a huge sin that must be avoided by Muslims.
Using his Twitter account on December 24, 2016, he wrote, “Wishing ‘Merry Christmas‘ to Christians is Worst Evil, Worse Than Fornication or Murder.
Astagfirullah… (Please Share)”
“Please avoid it my dear Muslim brothers and sisters.. It’s a big sin…. Please RT this and spread the message”, he added.
His argument for this statement is that doing this means Muslims agree that Jesus Christ was born on December 25th, which is Christmas day.
According to the scholar, this makes saying ‘Merry Christmas’ a SHIRK (the worship of anyone or anything other than Allah.)
“SHIRK IS THE GREATEST & THE ONLY UNFORGIVABLE SIN!!!” he added….