Fredo didn't know. He was simply not strong or brave enough to have gone so stoically to his end, as Tessio had.

The way I've always seen it is that Fredo is simply disappointed that Anthony cannot go fishing with him. Had he any idea what was about to happen, he wouldn't have so cheerily promised Anthony he'd catch a fish using their little secret prayer.

I never took his prayer to be 'loud'. I know people usually whisper their prayers or even silently move their lips - but that is not the type of praying Fredo was involved in. It was a ritual he used to catch a fish.

Anyway - we had to hear him recite it just that way for dramatic effect. Especially the cutoff of that final line (for those who know it)..now and at the hour of our death.

Apple