Scholars say it is not allowed to attend a non-Muslims funeral no matter how close he is (See for example islamqa #145532), this opinion is in first place the one of the Maliki and Hanbali madhhab, while the Shafi'i madhhab considers it as allowed for really close non-Muslims (one of their evidences is this hadith). They also consider attending the funeral ceremony at a church as a bigger sin.
Discussion of some of the given evidences are:
From the Quran:
And do not pray [the funeral prayer, O Muhammad], over any of them who has died - ever - or stand at his grave. Indeed, they disbelieved in Allah and His Messenger and died while they were defiantly disobedient. (9:84)
- the context of this verse is talking about apostates or hypocrites whom left the Muslims alone when the Muslims were in need of them!
- It seems at least to directly address our Prophet () so maybe this doesn't apply for Muslims in general, but could apply for a Muslim authority like a caliph etc.
It is not for the Prophet and those who have believed to ask forgiveness for the polytheists, even if they were relatives, after it has become clear to them that they are companions of Hellfire. (9:113)
might be considered as a more general ruling, but this seems to apply for living people in first place!
This hadith quotes the revelation circumstances of the 2nd Quran quote.
However he allowed or even ordered 'Ali ibn abi Talib to bury his father in this hadith.
I'd like to say if you fear for your belief by attending the funeral or if you are afraid to be counted a sinner you shouldn't go. If there's any chance that your presence could have a positive impact for Islam and the acceptance of Muslims I'd even recommend you and ask you to reject any act of shirk or disbelief you may witness. You must have in mind the intention is the key.
Also as @Honey said in her answer Islam is more than orders and prohibitions it's a social religion, where we are asked to interact with people, like neighbors, friends and relatives.
My answer so far is assuming the man was a non-Muslim!
Now there's an other Issue which is if your uncle is/was a Muslim he shouldn't be buried in a non-Muslim cemetery! And his funeral shouldn't be according a non-Muslim ceremony!
Burying a Muslim in a non-Muslim cemetery is considered as haram by consensus of the scholars according to this fatawa (islamqa #180576 only available in Arabic) one evidence for that is the hadith from sunan abi Dawod:
Narrated Bashir, the Client of the Messenger of Allah:
Bashir's name in pre-Islamic days was Zahm ibn Ma'bad. When he migrated to the Messenger of Allah (ﷺ). He asked: What is your name? He replied: Zahm. He said: No, you are Bashir.
He (Bashir) said: When I was walking with the Messenger of Allah (ﷺ) he passed by the graves of the polytheists. He said: They lived before (a period of) abundant good. He said this three times. He then passed by the graves of Muslims. He said: They received abundant good. ...
So be aware if this man really was a Muslim and he was buried in a non-Muslim cemetery and no body has prayed for him the salat al-Janazah the whole Muslims (at least those who knew about it) would be blamed and commit a big sin (read for example these fatwas islamqa #153492, #14040 and #20472)!
In case of doubt you must check what you think is the closest to truth and handle according what you concluded. Again unfortunately there's only an Arabic version a fatwa available on islamqa #130202, which deals about whether it is allowed to pray the janazah prayer for somebody who didn't pray (which is considered as equal to disbelief). If you have been certain about his disbelief you must follow the orders given by (9:84) which means not to pray janazah.
If you think he was a Muslim and didn't attend the funeral you have at least the option of the prayer in absence (salat al-ghaib) (for details read this fatwa islamqa #35853)