Is it permissible to congratulate non-Muslims on their religious festivals like Christmas, Diwali, etc.?
God Almighty says, “God does not forbid you from those who do not fight you because of religion and do not expel you from your homes – from being righteous toward them and acting justly toward them. Indeed, God loves those who act justly” (Quran 60: 8).
In Islamic law, there is no objection to congratulating and sharing peaceful non-Muslims their religious occasions that do not violate the fundamentals of Islam, especially if they are family members, relatives, neighbors, colleagues, etc. This is encouraged especially if they exchange congratulations with their fellow Muslims on the Islamic occasions as per the words of God Almighty, “And when you are greeted with a greeting, greet [in return] with one better than it or [at least] return it [in a like manner]” (Quran 4: 86).
Exchanging congratulations with non-Muslims does not mean acknowledging their disbelief but is from among the forms of righteousness and justice that God Almighty loves.
A Muslim is commanded to speak good words and treat others kindly in a manner that is conducive to loving Islam and introducing its tolerant nature and comprehensiveness. God Almighty says, “And speak to people good [words]” (Quran 2: 83). He also says, “And do not argue with the People of the Scripture except in a way that is best, except for those who commit injustice among them, and say, “We believe in that which has been revealed to us and revealed to you. And our God and your God is one; and we are Muslims [in submission] to Him” (Quran 29: 4).
As for the opinion of some scholars who maintain the prohibition of congratulating non-Muslims on their religious occasions, their prohibition only pertains to words which show acknowledgment of disbelief whether explicitly or implicitly, or to words that show acknowledgment of any of their invalid acts.
Based on the above, congratulating non-Muslims on their occasions and accepting invitations to attend their celebrations do not necessitate acknowledging their disbelief or corrupt convictions.
And God Almighty knows best