According to the usual narrative, in the half-century after the First Ecumenical Council, the doctrine of Nicaea – that which the Fathers of Nicaea had agreed in 325 - was retrieved by St Athanasius and others in the face of a backlash against the Nicene Creed. However, the journey to the establishment of the homoousion [of one substance] is not as straightforward as that account suggests. In the end, the homoousion did come to be the hallmark of Christian Orthodoxy, but that is not the same as saying that the faith of the Fathers of Nicaea was retrieved and upheld. In this paper, Professor Louth will address three questions: (1) What did the Fathers of Nicaea mean by homoousion to patri (i.e. of one substance with the Father), and why did they include that phrase in the Creed? (2) Whatever it meant then, what did it mean to those who saw in the decisions of Constantinople I a reaffirmation of Nicene Orthodoxy? (3) What does it mean by suggesting that the homoousion eventually became the hallmark of Orthodoxy?