Why Muslims Do Not Celebrate And Congratulate Non-muslims On Their Festivals?

No doubt, we have friends, teachers and colleagues among non-Muslims. We engage and collaborate when it comes to worldly affairs. We even congratulate them on their success. This is fine and humanly!

We have a lot of non-Muslim teachers we respect them a lot - we benefitted a lot from them in terms of knowledge and experience. We do not sever our relationship with them!

Also, we have a lot of non-muslims as friends and colleagues at work. We cherish and wish them well. We congratulate them when they achieve great things in life.

In essence, Islam enjoys being good to all the creatures of Allah - human beings, whether a Muslim or non-Muslims. Even animals!

However, when it comes to the issue of beliefs, Muslims are not to compromise. In Islam (as it is contained in other religions/beliefs), there are dos/don't and red lines we shouldn't cross. Some of them include things we are prohibited from doing such as celebrating any non-islamic festivals and wishing those doing them. This is not limited to non-Muslim festivals only. It also includes all the innovated festivals some Muslims celebrate, such as Mawlud Nabiyy, Islamic Calendar year, etc (you see some Muslims and I have made some threads condemning these too). For any festival to be Islamic, there must be evidence for it from the Holy book (Qur'an) or Sunnah! We celebrate both Eids because there are account for them!

Based on the above, congratulating the non-Muslims on their religious festivals is haraam because it implies that one accepts or approves of their rituals of kufr, even if one would not accept those things for oneself. 

Allah says (interpretation of the meaning):
"If you disbelieve, then verily, Allaah is not in need of you, He likes not disbelief for His slaves. And if you are grateful (by being believers), He is pleased therewith for you. . ."
[al-Zumar 39:7]

". . . This day, I have perfected your religion for you, completed My favour upon you, and have chosen for you Islaam as your religion . . ."
[al-Maa'idah 5:3]

So to congratulate non-Muslims on their festival is forbidden if you're truly a Muslim. You can congratulate them on any worldly achievements so far it has nothing to do with their beliefs. 

Also, it is haraam (forbidden) to accept invitations on such festivals, because this is worse than congratulating them as it implies taking part in their celebrations.

We ask Allah to make us steadfast, safe us hypocrisy and from displeasing our creator at the expense of His creatures (human beings) (Aameen).