Arminian Randal Rauser recently posed an attack on penal substitution:
Some Christians (advocates of penal substitution) believe that Jesus’ atoning death satisfies the wrath of the Father against sin, and thus that Jesus’ death provides the culmination and completion of the temple sacrificial system.
The difference can be illustrated with the standard story of the South Pacific islanders who believe an innocent virgin must be tossed into the mouth of the volcano to satisfy the Volcano God so that he will not erupt and thereby smite the people for their sins.
The advocate of penal substitution offers a view of divine wrath and justice which is in continuity with the framework of divine/human relations that is assumed by the South Pacific islanders. To be sure, the advocate of penal substitution does not commend the act, but he does share the logic: God is wrathful against sin and that wrath can be satisfied by an appropriate substitutionary sacrifice.
i) Tossing virgins into a volcano is a scurrilous comparison. Now, Rauser may feign that he's not directly comparing penal substitution to tossing virgins into a volcano, but he's clearly trading on the lurid connotations of pagan sacrifice to tarnish penal substitution by association.
ii) Penal substitution doesn't mean the Father's "wrath" is satisfied. Even if we operate with the "wrathful" framework, it's not as if the Father is wrathful about sin, while the Son (and Spirit) are not wrathful about sin. It's not as if an unwrathful Son placates the Father's wrath. Rather, it would be a case of placating divine wrath. It's not as if one member of the Trinity can be wrathful while the other two are not. If there is such a thing as divine wrath, the Trinity would be wrathful.
iii) Divine wrath is not a presupposition of penal substitution. Penal substitution concerns the satisfaction (through vicarious atonement) of divine justice, not divine wrath. Even if God wasn't wrathful, that wouldn't eliminate penal substitution. In fact, wrath cannot be satisfied. That's a category mistake. Wrath is not a forensic category. By contrast, justice can be satisfied. Justice and satisfaction are both forensic categories.
iv) I don't think God is literally enraged by sin. I think that's a colorful, anthropomorphic way of depicting God's disapprobation regarding sin.
Perhaps Rauser would say he's commenting on popular presentations of penal substitution. If so, that hardly disproves more precise formulations.
v) The OT, which provides the typology of penal substitution or vicarious atonement, forbad human sacrifice.
vi) Mere human sacrifice cannot atone for sin. Putting a sinner to death would fail to atone for even his own sin.
vii) Jesus is innocent in a way that no sinner is. Moreover, Jesus isn't merely human.