The composite nature of PSA explains why few Christians before the reformation defined PSA exactly as the Reformed did, while most pre-reformation Christians affirmed the first principles that would make up the doctrine. Hence, the historical plausibility of PSA derives from the fact that each of its theological first principles finds clear affirmation among Christians and Scripture. So its composite conclusion not only follows from these but also has its roots in 2,000 years of church history.

Penal substitutionary atonement (PSA) is not a single doctrine. As the name suggests, the doctrine comprises theological principles like a penalty for sin, a substitutionary saviour, and a particular vision of the atonement. And actually, it draws from even more theological first principles than this list. 

The composite nature of PSA explains why few Christians before the reformation defined PSA exactly as the Reformed did, while most pre-reformation Christians affirmed the first principles that would make up the doctrine. Hence, the historical plausibility of PSA derives from the fact that each of its theological first principles finds clear affirmation among Christians and Scripture. So its composite conclusion not only follows from these but also has its roots in 2,000 years of church history. 

These first principles include: God is just, we are unjust, and the man who ascended the cross substituted himself for us to bring us salvation. Put together, PSA makes good, biblical sense. And stated in this way, it is obvious how Christians throughout the ages have affirmed these biblical teachings using different idioms of theology. In the following, I explain these theological first principles, albeit in short form.

Justice

While interceding for Sodom, Abraham rhetorically asks God, “Shall not the Judge of all the earth do what is just?” (Gen 18:25). His point is that God as universal judge is just. God confirms this supposition by responding, “If I find at Sodom fifty righteous in the city, I will spare the whole place for their sake” (v. 26). We learn here that God will save the unrighteous through a righteous substitute since it would not be right to punish a righteous person—in this case, fifty such persons.

Scripture consistently presents God as a just judge. He gave the law to Sinai in which principles of justice lie out for all to see. When he reveals the meaning of his name through his pluriform attributes, he not only affirms his will to forgive but also that he will “by no means clear the guilty” (Exod 34:7).

His justice, however, works together with his patience and compassion. For his chosen people, he gave the law and sacrificial system to offer forgiveness, to encourage holiness, and to sustain fellowship with God and others. To gentiles, he not only offered Israel as a revelation of himself but also patiently passed by their sins to give full opportunity for repentance. 

He also distinguishes between willing and unwilling sins. The former primarily has punitive consequences while the latter seems more restorative since restoration to the community is the end-goal (Num 15:27–31). 

Christians, therefore, affirm the goodness of God’s justice. God rightly punishes us for our sins according to our works. He is neither too unjust nor too lenient; he often surprisingly will give the grace of forgiveness while at other times will administer justice directly. 

So God is just, and he so chooses to administer justice according to his divine will. He created us with the capacity for culture—agriculture, horticulture, politico-culture, and so on. So society forms on the basis of human capacity for order and community. God chooses to work through these human means to manifest his justice—through law codes, covenants, and so on. 

This divine accommodation, however, does not mean God allowed Israel to have wrong views about God or that God merely is an artifact of human culture. Within the culture of the world, God breaks in and establishes his kingdom whether through the idiom of ANE law codes or the New Covenant in his blood. 

The entire preaching ministry of the church brings about the miracle of salvation through human means and Spiritual application of these means. God just works this way. And it allows us to understand his justice from our human perspective. In this general sense, everything God does is anthropomorphic—done in a way to communicate to humans. God lovingly accommodates himself to us without compromising his Being or Truth. 

Unjust

We inherit sin from our original father, Adam. Death follows sin. Everyone dies. So it is obvious that everyone sins as Paul reasons in Romans 5. More than that, experience teaches us that nobody is free from sin. As anybody can affirm, evil acts should be punished. Good acts should be rewarded. So God, just rulers, and parents do just that. It follows that God should punish the perpetrators of evil for their wrongdoing. 

A murderer should be stopped. His victim should be vindicated. We sin and sin badly. Even from birth, sin threads through our bodies. David writes, “I was brought forth in iniquity, and in sin did my mother conceive me” (Ps 51:5). So we are sinful at birth, and we act sinfully throughout life. Both the son that we are born into and the sins that we commit warrant judgment. 

Still, it is hard to understand how an infant could be judged as a sinner and so condemned to eternal conscious suffering. And this is why many Christians affirm that God saves children and the mentally disabled when they die. Put positively, every baby who dies is elect. In practice, then, God does not condemn someone until they can be accountable for their own actions.

Some Reformed thinkers do affirm that unbaptized babies enter into eternal perdition. For example, Peter Vermigli pleads ignorance on how this works exactly, yet affirms that unregenerate infants will enter hell. He qualifies his assertion: “Since they did not add actual sins to their original sin, they will be punished more lightly”. He then adds: 

“Yet, I always except the children of the saints, because we do not hesitate to count them among believers, although they do not in reality believe yet because of their age. We do not reckon the children of infidels among believers, though in and of themselves they are not rejecting the faith. From this, believers’ children who have died without being baptized, because of the covenant that God struck with the parents, can be saved, if they are predestined as well. I will except any others too who are predestined by the hidden council of God” (“On Original Sin,” in Commonplaces, Ch 11).

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