In the third verse of his eponymous New Testament epistle, Jude exhorts his readers "to contend for the faith which was once for all delivered", a charge that continues to resound to the present day. This collection of essays responds to the apostle's call by providing both a diagnosis of the ills of modern progressivist Catholic doctrinal and moral theology and a prescription for the safeguarding of orthodoxy via a rightly understood return to the traditional sources of theology. The essays in the first part of this collection seek to answer the question, "What went wrong with Catholic theology since the Second Vatican Council?". Following a brief account of the movement in modern theology from its philosophical basis in Kant and Hegel to the nouvelle théologie and later progressivist theologies of the twentieth century, the writings of Karl Rahner, Walter Kasper, and Bernhard Häring are treated as representative of principal problematic trends, and the concept of heresy is surveyed as it has been understood in the past and as it operates in the Church today. The essays in the second part indicate the way forward for Catholic doctrinal and moral theology, examining and distinguishing the orthodox use of the fontes theologiae of magisterial teachings, the deposit of faith in its development, the "sense of the faithful" (sensus fidelium), Sacred Scripture, and Church councils and synods. In its twofold attentiveness to contemporary errors in Catholic theology and to tradition-based correctives, The Faith Once for All Delivered offers an urgent and compelling summons to the sacred mission of defending doctrinal and moral orthodoxy.