Grace, Predestination, and the Permission of Sin seeks to analyze a revisionist movement within Thomism in the 20th century over and against the traditional or classical Thomistic commentatorial treatment of physical premotion, grace, and the permission of sin, especially as these relate to the mysteries of predestination and reprobation. The over-arching critique leveled by the revisionists against the classic treatment is that Bañezian scholasticism had disregarded the dissymmetry between the line of good (God's causation of salutary acts) and the line of evil (God's permission of defect and sin). The teaching of St. Thomas is explored via intimate consideration of his texts. The thought of St. Thomas is then compared with the work of Domingo Bañez and the foremost 'Bañezian' of the 20th century, Reginald Garrigou-Lagrange. The work then shifts to a consideration of the major players of the revisionist treatment, including Francisco Marín-Sola, Jacques Maritain, and Bernard Lonergan. Jean-Herve Nicolas is also taken up as one who had held both accounts during his lifetime. The work analyzes and critiques the revisionist theories according to the fundamental tenets of the classical account. Upon final analysis, it seeks to show that the classical account sufficiently distances God's causal role in regard to free salutary acts and His non-causal role in regard to free sinful acts. Moreover, the revisionist account presents significant metaphysical problems and challenges major tenets of classical theism, such as the divine omnipotence, simplicity, and the exhaustive nature of divine providence. Finally, the implications of the traditional view are considered in light of the spiritual life. It is argued that the classical account is the only one which provides an adequate theological foundation for the Church's robust mystical and spiritual tradition, and in particular, the abandonment to divine providence.