Now that is giving witness out loud and in public! Plus it is beautiful to hear.
Click on the little "CC" in the red oval to see the English captions. Via Deacon Greg at The Deacon's Bench.
Fred in the comments elucidates further.
FYI, it is a common refrain in Eastern Orthodox Easter services... repeated over a hundred times in various forms. People in the middle east would recognize it like we recognize the "Hallelujah chorus" in the west. Its one of those that reaches across faith traditions, most people have heard it.This makes sense because you can see people joining in and singing from around the edges, though not so lustily as the main singers.