Why do the faithful have to say "And with your spirit " in the Catholic Mass? In the Catholic Holy Mass (Ordinary Form of the Mass in the Latin rite), the prayer  for peace is said more than once . But, when the priest says "Peace be with you"the faithful respond by saying "And with your spirit ", rather than saying "And with you". I wish to know  why  the faithful are required to pray for peace `with the spirit of the celebrant'.
The simple answer is that &quot;thy spirit&quot; is a Hebraism for &quot;you&quot; (to be specific in this case, &quot;you,&quot; but more 'endearing,' 'elevated,' or 'prayerful;' cf. Lk. 1:46). St. Paul uses this blessing in 2 Timthy 4:22: &quot;The Lord Jesus Christ be with thy spirit...&quot;
This was almost certainly imported into, or was always part of, the Roman liturgy very early, and it has remained in it ever since.
Indeed, it appears that all the apostolic Liturgies involve this Hebraism, explainable only by the primitive apolostic foundation each of the Liturgies have.
For example, the Liturgy of St. John Chrysostom, the most popular Liturgy in the East, one hears the priest say, &quot;Peace be with you all,&quot; and the deacon responding, &quot;And with thy spirit.&quot; The Liturgy of St. Mark (Alexandrian Liturgy) also has this, as does St. James' (Antiochian) Liturgy.
As to why the laity (or the deacon on their behalf) are to respond to the blessing of the priest, the theological reason is that at Mass, there is to be kept a close unity of offerer and the others who offer by him, the Eucharist as well as the sacrifice of their praise and prayers.
For example, in many Liturgies we find this common prayer or ones very similar:
Liturgy of St. Thomas/Malankra/Indian Rite

Priest. Pray, brethren, that our sacrifice may be acceptable to God, the almighty Father.
Deacon. May the Lord accept the sacrifice at your hands for the praise and glory of his name for our good, and the good of his Church.

Roman Liturgy of the Extraordinary Form of the Mass

Priest. Pray, brethren,  that my sacrifice and yours may be acceptable to God, the almighty Father.
Deacon. May the Lord accept the sacrifice at your hands for the praise and glory of  his name, for our good and the good of  all his holy Church.

The laity, via the Deacon (or servant), are to be always united and prayerfully attentive to the priest's intercessory prayer and offering of the Eucharist. That is why when blessed, they return the blessing, as it were: &quot;The Lord be with you: and with thy spirit [also]!&quot; eagerly awaiting what comes next. It involves the laity in the Sacrifice they are otherwise unable to offer themselves, via union with the priest who offers.