Over the last three decades there has been a great outpouring of writings from both Catholic and Protestant theologians on the doctrine of the Trinity, almost all of which, ironically, have lamented the neglect of the doctrine. Again and again one reads that although the Trinity is central and crucially important to Christianity and Christian theology, it has not been given adequate treatment. It is unacceptable, theologians protest, that the Trinity has come to be regarded as an obscure and complex theological technicality, a piece of celestial mathematics impossible to understand and with little relevance to the life of the ordinary Christian. Karl Rahner remarked that modern Christians were ‘almost mere “monotheists’” paying lip service to the Trinity but in practice ignoring it. If it were announced that the dogma had been a mistake and was to be erased from official Christianity, nobody, he thought, would be too bothered, neither the ordinary believing Christians nor the authors of theological textbooks.’ Rahner’s diagnosis has been widely accepted and widely regretted. The consensus is that the Trinity is at the heart of Christianity, and both theology and piety have gone astray if it is regarded as belonging to the specialists. A retrieval (it is believed) is needed: the Trinity must be understood once again (one reads) as a positive and central element in the Christian faith rather than an embarrassing obscurity, and as profoundly relevant to the life of individual Christians, to the life of the Church, and perhaps beyond.