I think the best time (and perhaps the only time) to sing a nationalistic song) is when you are not of the nation the song is celebrating or when you are of the nation the song may be against and when you are singing it in neither of these countries).

For example when an Englishman sings Flower of Scotland in Wales. And preferably sings it in Welsh...............

Possibly the best people to sing Gospel and religious music are non-believers?