Deuteronomy 27:23

‘If there is a man who marries a woman and her mother, it is lewdness; both he and they shall be burned with fire so that there will be no lewdness in your midst.

“Indeed all Israel has trespassed against Your law, even turning aside, not listening to Your voice; so the curse has been poured out on us, along with the oath which is written in the law of Moses the servant of God, for we have sinned against Him.

Otherwise if you bless in the spirit only, how will the one who fills the place of the uninformed say the “Amen” at your giving of thanks, since he does not know what you are saying?