Augustine’s remark, which Peter Lombard put on the Wrst page of his study of the Trinity in the Sentences,1 gives us the Xavour of Trinitarian reXection in the golden age of scholasticism. As St Thomas’ Master, Albert the Great saw the matter, precisely because it belongs to this Weld, to show the goal of human existence—making mistakes here will divest faith and theology of their purpose: ‘The whole of human knowledge comes to fruition in knowledge of the Trinity. For every science and every thing to which the mind applies itself is looking for that which gives us happiness. Speaking about other things is only worthwhile when it derives from and guides us to this search.

St Thomas would follow that up by saying that, ‘The whole of our life bears fruit (fructus) and comes to achievement (Wnis) in the knowledge of the Trinity.’3 This ‘knowledge of the Trinity’ is supplied by Christian faith, and so paves the way for the vision of the Trinity. It is the way to happiness: ‘The Lord taught that the knowledge that makes us happy consists in knowing two things: the divinity of the Trinity and the humanity of Christ.’4 Faith in the mystery of Christ enshrines and implies faith in the Trinity.5 Within the pilgrimage of faith made in the hope of happiness, the theologian’s vocation consists in giving an account of the mystery which he has received, after the pattern of 1 Peter 3.15, a verse which St Thomas loved to quote in order to describe the task to which he dedicated his life within the Order of St Dominic: ‘Always be prepared to satisfy everyone that asketh a reason for the hope and faith which are in you.