At the beginning of Chapter 11 I said that Aquinas thinks of the Trinity as something in whose life we may come to share. The Trinity has dropped out of my account of Aquinas's teaching since then, but we can now begin to connect what he says about people in general with his teaching concerning the Trinity. More precisely, we can now start to see why belief in the Trinity is, for him, more than what I earlier called 'a complicated bit of speculative celestial physics'. To put it very simply, his position is that the Trinity makes us divine since God, who is Father, Son, and Holy Spirit, brings us to the final or ultimate good or end of rational creatures, which is nothing less than God himself. For Aquinas, the Trinity is not just a fact from eternity, so to speak. It is the means of human redemption and deification. It is God in love with us making us his friends.
To appreciate what Aquinas intends by that suggestion we can start with his claim that God is the means by which we can be better than we are when considered as merely human. As we have seen, he thinks that people can attain a limited and partial happiness simply by being human. We can link up with Aristotle in the quest for human excellence, and we can fulfil ourselves to some degree by means of acquired moral virtues. As we have also seen, however, Aquinas does not think that this will give us perfect happiness. For that, he says, we need beatitudo, or a union of mind and will with God.
The ultimate happiness of people lies in their highest activity, which is the exercise of their minds. If therefore the created mind were never able to see the essence of God, either it would never attain happiness or its happiness would consist in something other than God. 1