>>210
Either it is the only church in town, or the church had been mentioned before.
Yes, but it seems that we can shorten this to "there is only one of something". For example, "I joined a group of warriors. The group of warriors was very brave". The "a" in the first sentence means that there wasn't only one group of warriors. The second sentence (methinks) is just a shortened form of "The group of warriors that I joined was very brave". There is only one "group of warriors that I joined", so we must use "the".
Am I not right?
"In a town there were many churches. One of the churches had a leaky roof. Some workmen went to the church to repair the roof, but they found it was impossible to repair without materials from the neighboring town 3 miles away."
It's also a shortened form of "In a town there were many churches. One of the churches I mentioned before had a leaky roof. Some workmen went to the church I mentioned before to repair the roof I mentioned before, but they found it was impossible to repair without materials from the neighboring town of the town I mentioned before 3 miles away."
There are many towns, so we use "a".
There is only one group of "churches I mentioned before", so we use "the".
There are many leaky roofs in the world, so we use "a".
There is only one "church I mentioned before", so we use "the".
There is only one "roof I mentioned before", so we use "the".
There is only one "neighboring town of the town I mentioned before", so we use "the".