We know that Allah has forbidden to destroy Churches and Temples etc. according to Verse of Surat al Hajj (22:40), but this doesn't mean that a Muslim should accept them as a Place where Allah is still worshiped according to this fatwa. But you should know that most Fatwa come from Scholars coming from countries where Islamic Thinking and Lifestyle is dominant so they are sometimes hardly applicable in non-Muslim countries! So if possible you should try to find some fatwas from local fatwa councils! In my Answer here i added a comment for a fatwa which considers the situation of Muslims living in non-muslim countries!
And as they are revealed in the Quran entering them isn't a sin, but what might be more interesting is the worship inside of them. As we know when 'Umar ibn al Khattab (May Allah be pleased with him) entered Bayt-al_Maqdis (Al-Quds/Jerusalem) to receive the Keys of the city he has chosen a place beside a Chruch to pray and thank Allah, so this is the most preferable thing to do as it seems to me!
But in some cases -in non Muslim countries- you might be in the situation that you should assist to a marriage, funeral etc. in a Church for example.
My advise will be you should over think if you really can reject going to the church without being impolite or going there.
As somebody who has a lot of non-Muslim relatives this situation could come to meet me more or less each year!
Well if i go to the church i only participate in matters which are acceptable
for me as a Muslim this means if i find in a text anything showing shirk i won't speak it out or sing it with them or if the preacher made a prayer (du'a) including shirk i won't say amin! As you might know there are many Quran Verses saying if the mushrikyn offend Allah you should leave the place until they change the subject! So this seems to me in that very situation the best way to do it by rejecting these false taughts in my heart!
And Allah knows best!