So say we all is an acclamation someone says when he or she affirms something to be true. It is analogous to "Amen."
"Our father's name lives on in another way now. Bill, you were named after your brother. This is our tradition. You will do the rest. As we are from the soil, so shall we return."
Bill drops some soil into a candle flame. Then everyone cheers him and says, So say we all. ("Apotheosis")