Idiom expressing a wish that the desires of the person to whom the saying is spoken will come true.
The original, literal meaning of the saying indicates that the speaker wishes that whatever the addressee has just said will be heard by God and answered.
May be used sarcastically, in which case it is roughly equivalent to "good luck with that."
John: Hopefully, my investments are going to return 400%!
Mike: From your lips to God's ears.