According to Hans Kung, Roman Catholic and Protestant theologians have reached fundamental agreement on the doctrine of justification. If this is so, then we can also expect to find such agreement on the doctrine of sin, for these two doctrines are but different sides of the same coin. This study tests this hypothesis by comparing Karl Barth's and Karl Rahner's views of sin. Fruitful comparison of Barth and Rahner is made possible by the material overlap in their theologies resulting from Barth's move away from Protestant liberalism toward a more orthodox theology and Rahner's move away from Neo-Scholasticism to a more critical theology. Comparison is made difficult by (1) the traditionally different points of departure of the Roman Catholic and the Protestant theology of sin, and (2) the difference between Rahner's transcendental method and Barth's narrative strategy. While Rahner founds his theology of sin on the concept of human responsibility, Barth takes God's effective act of redemption as the basis for his thinking about sin. Proceeding from the concept of responsibility, Rahner seeks the transcendental conditions of its possibility, but Barth begins with the story of sin's conquest by Jesus Christ, interpreting all other biblical material in its light. Despite their differences, Barth and Rahner essentially agree in all five areas in which they were compared. (1) They both argue that sin can be known truly only from the revelation of God. (2) Surprisingly, we find agreement in the cluster of issues surrounding the concept of freedom, Barth and Rahner agreeing that human beings have no neutral position vis-a-vis God, and that the sinful act is not free in the same sense as the obedient act. (3) They both describe sin as a three-fold "no": to God, to true human nature and to the neighbor. (4) For both theologians, the subject of the sinful act is the good human creature who is elevated by the address of God or the supernatural existential. (5) According to Barth and Rahner, sin results intrinsically in slavery and condemnation; sin is Hell, and Hell is sin.