After some opening reflections on the place of justification in recent Pauline scholarship, the essential argument is that both Catholic and Lutheran readings of Paul on justification, regardless of their strict exegetical accuracy, serve to articulate key principles of Christian existence under grace which need not only to be conjoined or placed alongside each other but need to be allowed, in the spirit of Receptive Ecumenism, to inform each other. In the Catholic case the need is for an expansion in the direction of a more effective and genuinely Pauline emphasis on the dynamism of grace. There are three sections. First, a detailed exploration, presented in a number of subsections, of the historic theological background to the issues at stake in the ecumenical dialogues and the way in which these might be best conceptualised. Second, situated against this backdrop is a summary exposition of some of the key achievements of the 1999 affirmation of the Joint Declaration on the Doctrine of Justification by the Lutheran World Federation and the Roman Catholic Church. Third, note is taken of some of the criticisms that have been made of the Joint Declaration, identifying the need for receptive ecumenical learning if its best intentions are to be realised. This generic need is illustrated in relation to one area of potential receptive Catholic learning from Lutheranism concerning the effective dynamism of grace and the character of Christian life as confident trust in this as a continually renewed event.