I wished Merry Christmas for my coworkers. Was this haram?

Intentions matter but unfortunately so do words especially those with meanings as such and unfortunately coming off as disrespectful isn’t enough of a reason to adhere to wishing someone Merry Christmas. As mentioned you’re better off saying happy holidays.

Better to just say happy holidays. Wishing merry christmas itself is worrysome as it is essentially adhering to the christian view of Isa AS. But I think your intention definitley matters. Just make tawbah and seek forgiveness where you have done any wrong.

May Allah forgive us. Intention is extremely important. And Allah knows best what is in our hearts.

Yes

You have associated partners with Allah. You became a kafir. Make tauba and recite shahadah again.

Allah knows your intention so I would not feel overly guilty about this, it is important also to get along with people and show that muslims are not aliens. For the future though, it would be good to have a few phrases up your sleeve as well wishes, "happy holidays", "enjoy your celebrations", "have a wonderful day" etc. These can be equally warm and acceptable without crossing your own personal boundary

By saying it you are saying that you are happy they claim God has a son. When someone wishes anyone a happy whatever or merry whatever, that person is saying they are happy for you. So are you happy they celebrate a day that they say son of God was born?

For everyone saying just say happy holidays, this doesn’t make sense to me. Since Merry means happy, and holiday in this context means Christmas. So you are saying the exact same thing. If someone wants to explain to me why saying merry Christmas is an issue. May Allah guide us