For Catholics and Orthodox, it is the failure to accept the finality of Christ's sacrifice and his role as high priest outlined in Hebrews. This manifests itself in Catholic attitudes towards the Mass, and exceedingly high view of Mary and the saints.
Could you elaborate on this?
Sorry for taking so long to get back to you on this.
Reformed theology teaches that Christ executes three offices, prophet, priest and king. This question concerns Christ's role as priest. Let's look at the Westminster Larger Catechism on the subject:
We get this from Hebrews 7 and 9
Christ died for us once, so to call the Eucharist, a sacrifice takes away from the sacrifice on Calvary. Likewise, since Christ is making intercession for us, to call Mary our co-redemptrix or to pray to the saints takes away from Christ's role as our redeemer and great high priest.
Since Christ has made one final sacrifice, and continually makes intercession for us as our high priest, Catholic attitudes about the Mass, Mary, and the Saints is unhelpful, or even idolatrous depending on how polemical you want to be.
I'm not sure you're accurately describing Catholic/Orthodox theology. A few points:
1) The Eucharist is not a sacrifice additional to Christ's but a participation in the eternal sacrifice on Calvary. Catholics believe Christ's body/blood is completely present in the Eucharistic gifts, the same body which was sacrificed on Calvary; we "re-enact" the breaking of Christ's body and shedding of Christ's blood and then choose to receive it, but this isn't viewed as a reenactment
per se as much as a literal revisiting to the Passion itself. It is necessary for us to accept this sacrifice won by the giving of Christ's body and voluntarily receive the grace conferred by His act; if such an acceptance were not necessary, then we verge on universalist territory.
A slightly separate issue is about the definition of "sacrifice" in this context. Christ's sacrifice is "The Sacrifice," as it were, but we all offer our own "sacrifices." Praise and thanksgiving are often referred to as "sacrifices of praise" or the like (even by most Protestants), of which the Mass is first and foremost. The existence of such sacrifices in no way undermines Christ's ultimate sacrifice.
2) The title of "co-redemptrix" is not an official Catholic title for Mary. It is not dogma nor doctrine and has always been controversial within Catholicism. To the extent that it is accepted, it's viewed in the following way: Mary saying yes to bare Jesus was a necessary condition for the salvation of the world for without it Christ would not have been born. Her life was invariably intertwined with Jesus's thereafter, and she consequently bore many of Christ's sufferings with Him (Mary was the original "redemptive sufferer" who unites their suffering to those of Christ's and thus in some small way participates in His act). However, she was not in any way sufficient in the redemption of mankind. Christ is the only Redeemer; Mary was a necessary assistance in performing His role as Redeemer. Whether that "takes away" from Christ's role is a matter of opinion; while I would agree that Catholicism places perhaps an overemphasis on Mary's role, I often feel Protestantism places far too little emphasis on her.
3) I'm sure you know Catholics don't "pray to saints" but ask for their prayers on behalf of us to God just as you might ask a friend to do so, and, that by virtue of their being in the direct presence of God, might have some additional "efficacy" in the transmission of their prayers. If you want to criticize this, I'd approach it more from the direction of questioning the eschatology necessary for saints to be with God rather than being raised on the last day or to what extent prayer's "efficacy" is an applicable idea. I view it as an optional thing that probably doesn't hurt, but then again I'm not as ingrained in Catholic culture as someone raised in it might be.
