Why was it necessary that the Son of God become a man? This question was most famously posed in the early church by Athanasius. He had focused on God’s equal commitments to the goodness of creation and the justice of the sentence of death for sin.1 Such a dilemma could only be resolved by one who took on “a body capable of death,”2 yet has life in himself so as to renew human nature.

At the turn of the twelfth century, Anselm of Canterbury argued that it had most to do with honor. Sin was an infinite dishonor to God—“sin is nothing other than not to give God what is owed to him”3— so that mankind had gotten himself into the worst sort of double trouble. Honor was still due, and yet offense is all that had been rendered, and, in fact, all that could be. 

A Crucial Distinction—Absolute Necessity and Consequent Necessity

When Thomas Aquinas addressed this same question over a century later, he introduced a category that would be used to great effect by later Reformed theologians. In responding to the objections that the incarnation was not necessary—that God could have restored humanity in other ways, Thomas said,

“A thing is said to be necessary for a certain end in two ways. First, when the end cannot be without it; as food is necessary for the preservation of human life. Second, when the end is attained better and more conveniently, as a horse is necessary for a journey. In the first way it was not necessary that God should become incarnate for the restoration of human nature. For God with His omnipotent power could have restored human nature in many other ways. But in the second way it was necessary that God should become incarnate for the restoration of human nature.”4  

Of course, that leaves the question as to why, in the second respect, the incarnation would have been necessary. Yet we must firmly understand this distinction first, which we may call the difference between an absolute necessity and a consequent necessity. Strictly speaking, an absolute necessity is one which carries its necessity in itself—the opposite state of affairs would be impossible in any possible world; whereas a consequent necessity is one which derives its necessity from another, that is, given some antecedent cause or reason, that which is consequently necessary is so in order that the antecedent may exist in a way consistent with its own possibilities. So it is here.

We turn now to John Calvin and Francis Turretin as representative figures among the Reformed to show continuity on this distinction.

Calvin on the Necessity of the Incarnation

In his Institutes, Calvin said that, “If the necessity be inquired into, it was not what is commonly termed simple or absolute, but flowed from the divine decree on which the salvation of man depended.”5

To anyone that would inquire beyond the decree, Calvin has sharper words:

“He who is tickled with a desire of knowing something more, not contented with the immutable ordination of God, shows also that he is not even contented with that Christ who has been given us as the price of redemption. And, indeed, Paul not only declares for what end he was sent, but rising to the sublime mystery of predestination, seasonably represses all wantonness and prurience of the human mind” (cf. Ephesians 1:4-7).6

A certain Lutheran theologian, Andreas Osiander, had applied himself to this very line of inquiry. According to Calvin, “Osiander ... has unhappily, in the present day, again agitated this question, which a few had formerly raised. He brings a charge of overweening confidence against those who deny that the Son of God would have appeared in the flesh if Adam had not fallen, because this notion is not repudiated by any passage of Scripture.”7 The Socinians would travel down the same path, though perhaps out of some different motives.

Calvin did not consider it speculative to ponder that which is necessary about the incarnation with respect to both the deity and the humanity of Christ. Something of each was revealed in Scripture. 

As to the divine side of the equation,

“It was his to swallow up death: who but Life could do so? It was his to conquer sin: who could do so save Righteousness itself? It was his to put to flight the powers of the air and the world: who could do so but the mighty powers superior to both? But who possesses life and righteousness, and the dominion and government of heaven, but God alone? Therefore God, in his infinite mercy, having determined to redeem us, became himself our Redeemer in the person of his only begotten Son.”8

And as to the human: “This will become still clearer if we reflect, that the work to be performed by the Mediator was of no common description: being to restore us to the divine favor, so as to make us, instead of sons of men, sons of God; instead of heirs of hell, heirs of a heavenly kingdom. Who could do this unless the Son of God should also become the Son of man, and so receive what is ours as to transfer to us what is his, making that which is his by nature to become ours by grace?”9

Turretin on the Necessity of the Incarnation

This necessity is argued by Turretin along the lines of three propositions:

1. “As the Son of God became incarnate only on account of sin, so it would not have been necessary for him to become incarnate if man had not sinned.”

2. “It was not only suitable, but necessary (sin and the decree of God concerning the redemption of men being supposed) that the Son of God should become incarnate in order to accomplish this work.”

3. “The work of redemption could not have been performed except by a God-man (theanthropon) associating by incarnation the human nature with the divine by an indissoluble bond.”10

How does he argue for these three propositions? To the first—that sin especially was presupposed—he gives three reasons: "(1) no other end of the advent of Christ and of his incarnation is ever proposed (whether in the Old or in the New Testament) than that he might save his people from sin ... (2) His office is occupied only with sinners ... In one word, he was incarnated that he might be a Mediator. And yet there would have been no need of a mediator, if there had been no sin ... (3) The impelling cause of the mission of Christ was the love and kindness of God toward men—not upright, but fallen (Jn. 3:16) ... (4) The fathers frequently confirm this."11  

Before giving his reasons for the second proposition, Turretin offers a threefold clarification on the state of the question:

“(1) The question does not concern a simple and absolute necessity on the part of God for God could (if he had wished) leave man no less than the Devil in his destruction. Rather the question concerns a hypothetical—whether the will to save men being posited, the incarnation was necessary; or whether it could have been brought about by some other means. (2) Again, the question does not concern the necessity of the decree for no one denies that on the supposition of God's having decreed this, it ought necessarily to have been done. Rather the question concerns the necessity of nature—whether (the decree being set aside and antecedently to it) it was necessary for the Son of God to become incarnate in order to redeem us. (3) The question does not concern the necessity of fitness because all confess this was in the highest degree fitting to the divine majesty—that his precepts might not be said to have been violated with impunity. Rather the question concerns the necessity of justice—that in no other way could the justice of God have been satisfied and our deliverance brought about (which we assert).”12

The state of the question being clarified, Turretin then argues for the proposition on two bases:

“(1) as God cannot deny his own justice, he could not free men without a satisfaction being made first. Satisfaction could not be made to infinite justice except by some infinite ransom (lytron); nor could that infinite ransom (lytron) be found anywhere except the Son of God ... (2) If it could have been done in any other way, it is not credible that the most wise and good God would have entered upon this counsel, which seemed to be little suitable to his wisdom and goodness ... And who can believe that the Father would have wished to send down from heaven his most beloved Son (in whom all his delight [eudokia] centers) unnecessarily to put on our flesh and to expose him to a thousand trials and at last to a most excruciating death for us?"13

As to the third proposition, concerning the two sides of the “God-man” relation, he wrote: “For since to redeem us, two things were most especially required—the acquisition of salvation and the application of the same; the endurance of death for satisfaction and victory over the same for the enjoyment of life—our mediator ought to be God-man (theanthropos) to accomplish these things: man to suffer, God to overcome; man to receive the punishment we deserved, God to endure and drink it to the dregs; man to acquire salvation for us by dying, God to apply it to us by overcoming; man to become ours by the assumption of flesh, God to make us like himself by the bestowal of the Spirit ... Therefore he ought to be between both and like Jacob’s ladder join heaven and earth by a participation of the nature of both.”14 

Concluding Remarks

Outside the context of a work of dogmatic theology, it would be easy to gather that the Reformed reflection on the incarnation was more rigorous in its logic over the order of salvation, yet not so much when it comes to the essence of God Himself. Or else, where God in Himself is considered, it is a voluntarist rather than realist assumption guiding it. That would be a faulty reading.

For John Owen, when considering the “discharge of the mediatory office by the Son of God, the Scripture most eminently represents his love, as the sole impelling and leading cause (Gal. 2:20; 1 John 3:16; Rev. 1:5).”15

Stephen Charnock puts the stress of the “first spring” of the cause in the will of God:

“Suppose God might have pardoned sin, and recovered man by his own absolute prerogative, had not his word been passed [if God had not said] that, in case of man’s transgression, he should die the death … Yet God chooseth this way, and is pleased with no other contrivance but this, and in a way of sovereignty he culls out his Son to be a sacrifice.”16

From this Charnock cites a number of passages on God commanding and Christ obeying that command (Isa. 42:1; Jn. 10:18; 14:31; 17:4; Phi. 2:7), so as to show that the focus is the divine will. And even when the emphasis is on the voluntary love of God, yet this love must meet the demands of His justice, as Charnock goes on to describe this justice as “righteously exacting obedience from his rational creature,” so that “his justice as a Ruler demands the punishment due for the transgression to be inflicted upon the offender.”17 If anyone doubts whether Charnock’s emphasis on the divine will as the supreme cause takes into account the whole essence of God—i.e., that it is not raw voluntarism, upholding the free decree in abstraction from the rest of the divine attributes—one need only read his Discourses on the Existence and Attributes of God with respect to the eternality and immutability of the decree.

It is true that among the Reformed, the voluntary condescension of God was more emphasized than it had been in previous centuries. Here the focus was either on the sovereignty of God per se, or else the satisfaction of God’s justice from the logic of passages like Romans 3:21-27, or perhaps the love of Christ in particular as our Great High Priest who sympathizes with us—here Hebrews 2:14-18 is a “necessity passage” which makes this the framework—yet there is nothing unique in these emphases that were not set forth by the early church fathers or even some scholastics. 

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1. Athanasius, On the Incarnation, II.6-8.

2. Athanasius, On the Incarnation, II.9.

3. Anselm, Cur Deus Homo? I.11.

4. Thomas Aquinas, Summa theologiae, Pt. III. Q1. Art.2.

5. John Calvin, Institutes of the Christian Religion, II.12.1

6. Calvin, Institutes, II.12.5.

7. Calvin, Institutes, II.12.5.

8. Calvin, Institutes, II.12.2.

9. Calvin, Institutes, II.12.2.

10. Francis Turretin, Institutes of Elenctic Theology, II.13.3.3, 14, 19.

11. Turretin, Institutes, II.13.3.4-7.

12. Turretin, Institutes, II.13.3.14

13. Turretin, Institutes, II.13.3.17

14. Turretin, Institutes, II.13.3.19, 20

15. John Owen, The Glory of Christ (Ross-shire, UK: Christian Focus Publications, 2004), 108.

16. Stephen Charnock, Christ Crucified (Ross-shire, UK: Christian Focus Publications, 2013), 30.

17. Charnock, Christ Crucified, 32.