As the great Jesuit liturgist Fr. Josef Andreas Jungmann wrote:
"We can best understand the (excellent link>) 'Et cum spiritu tuo' as a popular consensus in the work of the priest, not that the congregation here gives the priest authority or power to act in its stead, but that the congregation once more acknowledges him as the speaker under whose leadership the united group will approach almighty God. Thus in the greeting and its response we have the same double note that reappears at the end of the oration [opening prayer]; the 'Dominus vobiscum' seems to anticipate the 'per Christum' of the close of the oration, and the 'et cum spiritu tuo' is a forerunner of the people's agreement expressed in the Amen" (The Mass of the Roman Rite, Volume 1, Page 365).
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