“Existential risks” are risks that threaten the destruction of humanity’s long-term potential: risks of nuclear wars, pandemics, supervolcano eruptions, and so on. On standard utilitarianism, it seems, the reduction of such risks should be a key global priority today. Many effective altruists agree with this verdict. But how should the importance of these risks be assessed on a Christian moral theory? In this paper, I begin to answer this question – taking Thomas Aquinas as a reference, and the risks of anthropogenic extinction as an example. I’ll suggest that on a Thomist framework, driving ourselves extinct would constitute a complete failure to fulfil our God-given nature, a radical form of hubris, and a loss of cosmological significance. So the reduction of such risks seems vital for Thomists, or Christians more broadly. Indeed, I end with the thought that these considerations are not parochially Christian, or indeed specifically religious: everyone with a similar view of mankind – as immensely valuable yet fallen and fragile – should agree that existential risks are a pressing moral concern.