Tradition in Act addresses the present crisis over a proper understanding of the development of doctrine in contemporary Catholic theology and thus is a critical as well as constructive theological intervention in arguably the most central controversy in contemporary Catholic theology. The concept of “tradition in act” provides an approach that enables a recovered understanding of the development of doctrine for the theologian, and one that avoids the Scylla of antiquarianism and the Charybdis of presentism. The concept of “tradition in act” (drawing on the distinction between act and potency) is understood as the active undergoing of the continuously unfolding of the History of Salvation in the Church by way of the living paradosis of the doctrina evangelii in the Eucharistic liturgy, the life, and teaching of the Church. If doctrine is to develop at all, it must do so as the unfolding of a revelation once delivered to the apostles and entrusted to the Church as a sacred deposit. Hence it is required that subsequent formulations, however diverse in expression or circumstance, remain one with their antecedents—homogeneous with them, cognate to them in meaning and intention. This necessary continuity is identity. Without such identity, development degenerates into alteration and hence corruption; the line of transmission is broken, and the Church no longer hands on what she has herself received. The unfolding argument utilizes both the work and the approach of John Henry Newman to propose the criteria of identity of doctrine understood in terms of St Vincent of Lerins’ second rule, as presented by Newman, as constitutive of authentic development. The notion of authentic development of doctrine understood in terms of “tradition in act” offers a novel approach that avoids entirely the inherent risks in the historical consciousness of how doctrine has developed, in short, it is meant to offer an antidote to the historicism at large. Tradition in Act assays a contemporary Catholic theological articulation of the development of doctrine in the spirit of Gustav Mahler’s famous statement that “Tradition is not the worship of ashes, but the preservation of fire.”