Many answers to this purpose question are possible, of course, but I have found that the most illuminating, and often the most surprising, response that I can give is an argument based on the thought of the great medieval and scholastic thinker, St. Thomas Aquinas, who lived in the 13th century. It is a theological argument for the fittingness of evolutionary creation.
St. Thomas was a Christian theologian whose most mature work, called the Summa Theologiae, remains a masterpiece of faith-seeking understanding. In his writings, he frequently used theological arguments for fittingness to reveal the meaning, beauty, and wisdom of God’s actions in the world. Arguing from fittingness involves understanding why an end is attained better and more conveniently with the choice of one particular means rather than another. In this sense, and as St. Thomas himself explains, choosing to ride a horse is more fitting than walking if one seeks to quickly reach one’s destination on a journey (cf. Summa Theologiae, III.1.2). Theologically, arguments from fittingness try to explain how God’s choice of a particular means allowed him to most appropriately attain the end of his actions.
It is important to acknowledge at the outset that theological arguments from fittingness are not demonstrative. In other words, they cannot prove that a certain conclusion necessarily has to be the way that it is. They cannot prove that the conclusion is true. It may be fitting for someone to ride a horse to reach his destination, but he may in fact have chosen to walk instead. Theological arguments from fittingness do not prove doctrine. They attempt to reveal the inner coherence and the wisdom of the divine design, the theo-drama that has been revealed by a God who is true, good, and beautiful. Nonetheless, these arguments have been deployed by Christian theologians throughout the history of the Church to illustrate the coherence, the intelligibility, and the beauty of the Christian faith. For example, the author of the Letter to the Hebrews argues that it was “fitting [επρεπεν] that God, for whom and through whom all things exist, in bringing many children to glory, should make the pioneer of their salvation perfect through suffering” (Heb 2:10; NRSV).
Returning to our topic, we can reword the question as follows: Why is it fitting for God to have created via an evolutionary process rather than via special creation?
To answer this question, recall that for St. Thomas, theological arguments from fittingness attempt to explain how God’s choice of a particular means allowed him to most appropriately attain the end of his actions. Therefore, to grasp my argument for the fittingness of God’s creating via evolution, we need to begin by identifying the end of creation. Why did God create?
The Catholic theological tradition, which is representative of most of the other Christian traditions, has a clear answer to the purpose-of-creation question: God chose to create because he wanted to manifest and to communicate his glory. The Catechism of the Catholic Church, the definitive summary of Catholic doctrine, proclaims that, “Scripture and Tradition never cease to teach and celebrate this fundamental truth: ‘The world was made for the glory of God’” (no. 293).
How does God communicate his glory to his creatures? According to St. Thomas, God communicates his glory to his creatures by inviting them to participate in his existence. Creatures exist because God, whose essence is existence itself, gives them a share in his existence. This is the fundamental metaphysical distinction that distinguishes the Creator from his creatures: he has existence by nature, while they have existence by participation.
However, St. Thomas also explains that God shares his perfections with his creatures by inviting them to participate in his causality, which in the world manifests itself in his governance of his creation:
But since things which are governed should be brought to perfection by government, this government will be so much the better in the degree the things governed are brought to perfection. Now it is a greater perfection for a thing to be good in itself and also the cause of goodness in others, than only to be good in itself. Therefore God so governs things that He makes some of them to be causes of others in government; as a master, who not only imparts knowledge to his pupils, but gives also the faculty of teaching others. (Summa theologiae, I.103.6)
To put it another way, according to St. Thomas, it is a greater perfection, and therefore, more fitting, for God to share his causality with his creatures, making them authentic causes that can cause by their own natures, than for God to remain the sole cause acting within creation.
As I have explained to my students at Providence College, it is easy for human beings to write a book, but it is impossible for them to make a book that writes itself. On the other hand, God not only causes, but also creates creatures who are in themselves true causes. As such, when God does create creatures who themselves can cause, he manifests his power in a singular manner that signals his omnipotence.
Building upon this Thomistic theological account, I propose that it was fitting for God to have created via evolution rather than via special creation because in doing so, he was able to give his creation – the material universe and the individual creatures within it – a share in his causality to create. In this way, he more fully communicates his perfection to his creation, thus, more clearly manifesting his glory. As St. Thomas points out: “If God governed alone, things would be deprived of the perfection of causality. Wherefore all that is effected by many would not be accomplished by one.” (Summa theologiae, I.103.6)
Note that this is not the causality that allows one to create from nothing, because this causality is the sole prerogative of God who alone is creator. Rather, it is the causality that allows one to create novelty and diversity from pre-existing matter. This is also not the causality that philosophers call primary causality. Again, this is the sole prerogative of God who is able to act solely on his own power. Instead, it is the causality called instrumental causality where God, the primary cause, activates the instrumental causality of his creatures so that he and they can act together wholly and fully, to create, in the same way that an author and his pen work together wholly and fully to write a letter. This is the kind of causality that underlies biological evolution.