79 The august sacrifice of the altar is, as it were, the supreme instrument whereby the merits won by the divine Redeemer upon the cross are distributed to the faithful: "as often as this commemorative sacrifice is offered, there is wrought the work of our Redemption." This, however, so far from lessening the dignity of the actual sacrifice on Calvary, rather proclaims and renders more manifest its greatness and its necessity, as the Council of Trent declares. Its daily immolation reminds us that there is no salvation except in the cross of our Lord Jesus Christ and that God Himself wishes that there should be a continuation of this sacrifice "from the rising of the sun till the going down thereof," so that there may be no cessation of the hymn of praise and thanksgiving which man owes to God, seeing that he required His help continually and has need of the blood of the Redeemer to remit sin which challenges God's justice.
Source: Mediator Dei (Vatican.va)