When we look for an explanation of how life came to exist on earth, we want not just descriptions or a few reasons why life happened. We want to know about the causes for what has happened. Any theory of evolution must depend on a theory of what kinds of causes exist and of how these causes operate. If theistic science investigation proposes a new (and plausible) account of how causes operate in the world, we are obliged to reconsider the theory of evolution and revise it to take into account the proposed kinds of causes and their manners of operation.
Darwin, in fact, was motivated to develop his theory of evolution via natural selection because he precisely wanted to follow the then new naturalistic theory of causation. It was the theory in which God was not involved in the day-to-day running of the universe. From an early stage, Darwin looked for a theory in which a self-sustaining and self-developing natural world could produce all the living creatures seen today without God being responsible for its details and (in particular) not being responsible for the disease, predation, and parasitism that he saw. We may debate whether or not Darwin’s theory is plausible or successful with its causal explanations. However, within the absent-God causal scenario, it is clear that it is more or less the only possible explanation. As a result, it has today a very large number of followers. Many of them are still seeking the detailed causal explanations but uniformly agree on the ‘sanctity’ of the laws of nature within a naturalist philosophy.
There are many scientists who do profess religion and think that theism and Darwin’s theory can co-exist. This compatibility is possible since theism means to them that God sustains the world, and Darwin has described how creatures in the world have functioned and developed together. (They remind us of Galileo’s phrase, “Scripture teaches us how to go to heaven, not how the heavens go.”) This view, however, is equivalent to deism, not theism. It holds that God is not involved with the world once its operation has started (except, perhaps, in special events such as the founding and/or culmination of new religions). Once ‘laws of nature’ are assumed to be inviolate, Darwinism can accommodate such deistic views.
Within our new scientific theism we are unable to follow Darwin, in either the naturalistic or deistic world views. When God sustains the universe, this is not accomplished ‘at a distance’ by ‘merely sustaining’ the universe according to laws of physics but (we now conclude) by the presence of God in some degree. There can be no power without substance and no substance without present existence. This means that any sustaining action of God in the world will necessarily require the reception of life from God, not abstractly but as a substance really existing. This life is not always according to fixed physical laws. It necessarily has spiritual and mental components that will be effective if a suitable receptive form (e.g. a human form) is present. The fitness of a living organism is not purely a function of its interactions with the physical world and other organisms. It depends also and at least on the fullness of its reception of life from God. This implies that, within a proper theism, it is impossible to have a purely naturalistic account of evolution. Fitness, and hence selection, are not entirely natural. They are subject also to spiritual and mental considerations. There must be some kind of theistic selection, as well as natural selection.