Where did the apostle Paul die?
The Bible does not say, but tradition holds that he was martyred in Rome.
Opinion I would say Apostle Paul wherever I use 'Apostle' as a title, in the same way as I would say Mr Smith, with 'Mr' capitalised. Less frequently, I would write 'apostle' uncapitalised if I did not intend it as a title, for example where 'apostle' is the subject of the sentence, and Paul is addressed. Use capitals to be on the safe side.