Although he was a devout Catholic Byrd nevertheless produced music for his royal protector's church. Not very much of it to be sure and none of it was published under his name, people sometimes assume that because he believed so strongly that the Anglican church was in error that the music he produced for its services must necessarily be somehow lesser than the music he produced for his fellow Catholics and their persecuted church. Nothing could be further from the truth, as the Magnificat from the Great Service testifies.
It's a relatively late work that must date from the last years of the century and both its musical breadth and its lavish ten-part structure (SAATBSAATB) makes me think that he wrote it for the Chapel Royal. He pays musical homage to the previous generation's canticle settings but handles them in a far more imaginative and sophisticated way. There's a lot of juxtaposition and contrast which Byrd exploits to dramatic effect. The effect is both sonorous and beautiful I cannot help but think that Elizabeth would have been very pleased with him. Enjoy :-)