I know I've written a lot about Penal Substitution lately, but I believe I've come up with yet another devastating proof against the Protestant heresy of Penal Substitution which I must share. Don't worry, this will be brief.
Basically, my argument is that the doctrine of Penal Substitution makes God the Father the High Priest, which is impossible for two reasons. First, the High Priest offers sacrifice up to God, and yet if God the Father is 'at the top' of the hierarchy of existence already, then there can be nobody left to offer sacrifice to. So philosophically it's nonsense to say God the Father is or could be the High Priest. Second, the Bible plainly says that Jesus is the High Priest (Heb 2:17; 4:14-15; 8:1, etc), and as part of the Incarnation.
According to the error of Penal Substitution, the innocent person or animal receives the full punishment the sinner deserves. In the case of the Levitical Sacrifices, the animal was put to death, and Protestants see this as the Priest inflicting the punishment upon the animal which the sinner deserved. Ignoring the fact that putting the animal to death wasn't even an essential job of the priest for making atonement (since the sinner could put the animal to death, Lev 4:4-5; 4:15-16; 4:24-25), even Protestants agree that as High Priest Jesus was never pouring divine wrath upon Himself. So the Protestant is ultimately in a bind: they cannot say the Father was inflicting the punishment when inflicting the punishment was the job of the High Priest. (Note that "inflicting the punishment" is the Protestant view, not the Catholic one.) This problem is compounded by the fact that on the Day of Atonement, it was the High Priest who 'imputed the sins' (using Protestant terminology) onto the scapegoat, which likewise does not parallel the Protestant idea of God the Father 'imputing our sins' onto Jesus.
The only 'escape' for the Protestant is to say something to the effect that Jesus and the Father were effectively acting as one person, but that's the heresy of modalism, collapsing the Three Persons of the Trinity into one person. Only Jesus suffered and only Jesus was High Priest. PSub causes Christ's High Priesthood to fall by the wayside into irrelevance, in more ways than one.