Here the Council of Trent interposed with a definition of faith (Sess. XXII, can. iii): “If any one saith, that the Mass is only a sacrifice of praise and thanksgiving. . . but not a propitiatory sacrifice; or, that it profits only the recipient, and that it ought not to be offered for the living and the dead for sins, punishments, satisfactions, and other necessities; let him be anathema” (Denzinger, n. 950)
http://www.newadvent.org/cathen/10006a.htm
One Priest (Christ) and one Sacrifice that is both once and eternal.
NOW, THAT SAID: This has nothing to do with the original post. Please quit threadjacking.
http://www.fisheaters.com/mediatordei.html
Participating in mass is necessary for salvation. Christ did not finish His work on Calvary, humans must do it for Him. The mass is just another work in order to be saved
77. This purchase, however, does not immediately have its full effect; since Christ, after redeeming the world at the lavish cost of His own blood, still must come into complete possession of the souls of men. Wherefore, that the redemption and salvation of each person and of future generations unto the end of time may be effectively accomplished, and be acceptable to God, it is necessary that-men should individually come into vital contact with the sacrifice of the cross, so that the merits, which flow from it, should be imparted to them. In a certain sense it can be said that on Calvary Christ built a font of purification and salvation which He filled with the blood He shed; but if men do not bathe in it and there wash away the stains of their iniquities, they can never be purified and saved.