There is nothing wrong in saying "Merry Christmas", as you are wishing the person in front of you, or send a greeting to everyone else for that matter wishing them a good time and the happiest of tidings in Christ. Wishing someone well, good tidings and prosperity is never a bad intention.

 What could go wrong if someone approached you and said "Happy Ramadan" during the Ramadan month, not knowing or not paying attention or care that you might not celebrate the occasion? Wishing someone well, humility and peace is never a bad intention.

 Suppose someone is wishing you "Happy Hanukkah", without realizing you are not of the Jewish faith. Wishing someone well, a miracle and blessings is never a bad intention.

 There certainly are other well wishers that are suddenly approaching you- what will you do?

 Kwanzaa wishers want you to have great heart and celebrate the African heritage, the culture and the achievements of the black men and women throughout times. Specifically, those "first fruits of the harvest" ("matunda ya kwanza's" meaning) are honored and the African struggle is observed for one week during the holiday, certainly during which you may find yourself blessed with a greeting wishing you well, prosperity and reverance, which is never a bad intention.
 
 During the Diwali celebrations, the people of Hindu faith, or everyone of the Budhist faith are extending their illuminating attention which is meant to wish you well and to tell you that good triumphs over evil inside yourself everytime.

 We do see a pattern, don't we? Everyone is wishing you well, as if in their own language... How about in their own culture? How about in their own way? Or how about in the only way they have been brought up to say it?

 Anyone wishing you "Have a Merry Christmas and a Happy Kwanza, and Happy Hanukkah to you as well!" is gravely misunderstanding what they just said! During a holiday, the same could be said for the "Happy holidays" general wish as well, for they do not seem to realize, or they are not sure what to do or say in the respective holiday timeline... "Christmas is approaching everyone! Quick, panic!!"
 
 Basically, you can't go wrong with any of the wishes. It is with great satisfaction and in great heart allowed to say Merry Christmas, because Christmas is on the horizon and you are of Christian faith and you are wishing someone the best.

 However, as you have been reading, and as you are well aware of the different religions around the world, there is not a unifier, yet there are lots of dividers (even in Christianity) placed by mankind in order to sound more boastful of their own kind or to exalt one's religion in the eyes of God.

 Romans 3 is such an enlightening chapter, a highlighter of the truth under His watchful eyes, so here is a passage from it:

  Rom 3:21-27 "But now the righteousness of God apart from the law is revealed, being witnessed by the Law and the Prophets, even the righteousness of God, through faith in Jesus Christ, to all and on all who believe. For there is no difference; for all have sinned and fall short of the glory of God, being justified freely by His grace through the redemption that is in Christ Jesus, whom God set forth [as] a propitiation by His blood, through faith, to demonstrate His righteousness, because in His forbearance God had passed over the sins that were previously committed, to demonstrate at the present time His righteousness, that He might be just and the justifier of the one who has faith in Jesus. Where is boasting then? It is excluded. By what law? Of works? No, but by the law of faith."

 In conclusion, Christ has come to unify the world's people as well as the world's faiths and religions. He has come so that everyone who believes in Him may have everlasting life, for He is the way, the truth and the life, and no man or woman could come to redeem or reclaim
me or you or everyone else because we are, first, sinners, and, second, we are blessed, so we cannot bless ourselves but the Word of God and God Himself can, through His Word.
 

And a Merry Christmas to each and everyone of you as well!!