Q. 1. On special occasions, if invited by the priest to participate, can the faithful join in the prayer "Through him, with Him, in Him..."?
A. 1. No, the faithful may never participate in this prayer that has always been reserved for the priest. On this matter, "The General Instruction on the Roman Missal (GIRM)" states:
# 151. "After the consecration when the priest has said, Mysterium fidei (Let us proclaim the mystery of faith), the people sing or say an acclamation using one of the prescribed formulas.
At the end of the Eucharistic Prayer, the priest takes the paten with the host and the chalice and elevates them both while alone singing or saying the doxology, Per ipsum ('Through Him, with Him, in Him"). At the end the people make the acclamation, 'Amen.' Then the priest places the paten and the chalice on the corporal."