Is it allowed for us as Muslims to congratulate Christians in their eids? (I.e. Merry Christmas, etc.)

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, is His Slave and Messenger.

It is forbidden for all Muslims to congratulate any non-Muslims for their feasts. Allah Says (what means): {This day I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion} [Quran 5:3]

In addition, congratulating them for this feast is synonymous to accepting their wrongdoing and disbelief in Allah. Is there a greater disbelief than attributing a son to Allah or saying that Jesus is Allah or that Allah is the third of three “gods”?

Congratulating them also reflects accepting their rites and rituals, which are nothing but disbelief. Ibn Al-Qayyim said: “Congratulating the non-Muslims for their rituals is forbidden according to the agreement of all the scholars like: congratulating them for their feasts and fast by expressing good wishes: “Happy feast or enjoy your feast…” etc. If the Muslim who says this does not become a disbeliever himself, he, at least, commits a sin as this is the same as congratulating him for his belief in trinity, which is a greater sin and much more detested by Almighty Allah than congratulating him for drinking alcohol or killing a soul or committing fornication or adultery…etc'.

The one who commits such an act, has indeed committed an ugly and repulsive deed. Any Muslim who congratulates a person for a sin, an innovation or an act of disbelief has exposed himself to the wrath and anger of Almighty Allah.

Finally, if they (non-Muslims) congratulate us for our feasts, we do not congratulate them for their feast as we are on the right path and they are in error.

Allah Says (what means): {And whoever desires other than Islam as religion-never will it be accepted from him, and he, in the Hereafter, will be among the losers.} [Quran 3:85]

Allah knows best.