This article uncovers the roots of the doctrine of the Trinity in the 'prototrinitarian grammar of discourse on God' of the New Testament and in its Old Testament presuppositions. Contrary to the well-worn thesis of Harnack, it is argued that it was Jerusalem rather than Athens—i.e., the biblical witness rather than Greek metaphysics—that gave rise to the dogma of the Trinity. Greek metaphysics only came in when the early Christians had to express the universality of the truth they claimed for God's self-disclosure through Christ in the Spirit by engaging with Greek philosophy. This was a risky experiment, since it implied a conceptual redefinition that went against the doctrine's original import. It is shown, however, that the crucial link to the biblical witness was re-established by the Cappadocian fathers and subsequently adopted by the Council of Constantinople (381).