Certainly there is a real sense in which the Atonement is substitutionary in the Catholic mind: For in the Sorrowful Mysteries, we are encouraged to think on Christ bearing the sufferings for our sins, the punishment and death that we deserve. “He himself bore our sins in his body on the tree, that we might die to sin and live to righteousness” (1 Peter 2:24). “He is the true Lamb who has taken away the sins of the world; by dying He destroyed our death, and by rising restored our life” (Preface I of Easter). But the idea that Christ atones for our sins by satisfying or appeasing the wrath of God that would otherwise be poured out on sinners is just as surely contrary to everything we believe. If anything, in our mind, it is not God punishing Christ as we ourselves punishing Him through our sins. In the liturgy of His Passion, even, we the Church read the voices of Christ’s persecutors. Christ suffers for our sins, not because God pours out His wrath on Him, but because He in His Divine Mercy and love chose to take them on Himself.
This idea of “penal satisfaction” stands opposed to the very idea of the Mass: In the Mass, we re-present the eternal sacrifice of Christ, together with the sacrifice of ourselves, to the Father — because this sacrifice is pleasing to Him, an act of total, self-emptying love, an act of worship; not because it satisfies His wrath. Christ “gave Himself up for us, a fragrant offering and sacrifice to God” (Ephesians 5:2). We who share in His Body and Blood are partners in His altar (1 Corinthians 10:16–18), participating in His sacrifice, offering ourselves as living sacrifices (Romans 12:1).
But I think I can understand in this one of the reasons why some Protestants have such difficulty understanding and accepting the Mass, supposing that we are “re-sacrificing” Christ again and again, repeating His once and for all sacrifice (Hebrews 9:26). In their conception of the Atonement, because its primary purpose was to pay the penalty for our sins, and because He paid once and for all the penalty for all, for all times — because He satisfied the wrath of God once and for all — that sacrifice never has to be presented again. And in their minds, the very idea of re-presenting Christ’s sacrifice implies that we believe the wrath of God has returned and must be appeased again, that in our sins we have once again stirred His enmity. But we believe no such thing: In the Passion, Christ poured himself out wholly for us in love, a love that continues to flow, that is everlasting and never runs out; and in the Mass we continually join with Him in that love, in communion, in pouring out ourselves and offering ourselves wholly to God.
Addendum: I think, too, this might be a reason why Protestants misunderstand the Crucifix, the depiction of Christ “still on the Cross.” They object because this implies to them that we believe the work of the Cross, of the Atonement, is not finished; that Christ must continue to suffer again and again for our sins. But though His saving work on the Cross, the breaking of His Body and shedding of His Blood, is complete, He pours Himself our for us in love forever, a work that is never-ending. The grace, the love, mercy which flow from the Cross, will never cease to flow.