It’s
not a “Protestant” thing. It is found in the
Didache (
Catechism 2760) and many early liturgies. It’s still used (in one form or another) in many present Eastern liturgies. The fact that it is used by Protestants
may have been a factor for its introduction to the Roman Rite. I don’t think ecumenical intentions towards the Orthodox would have warranted its inclusion, since it was already present in Eastern Catholic Rites, and there’s no reason to refashion the Roman Rite to make it Eastern when it’s
not an Eastern Rite.
It
is not part of the
Our Father. It is the response to the priest’s embolism
after the
Our Father.