Niebuhr argues that human nature is uniquely dual. Humans are finite creatures, whose brief lives on earth are subject to the demands of nature and to their personal struggle to satisfy their individual needs. Through the power of reason, humans can choose how they live and think; they therefore have some control over their lives, but because they cannot escape their egoism and self-interest, conflict inevitably arises between individuals, social groups, and nations, even between humans and nature. In Christian terms, humans live in a state of sin as long as their actions are guided by self-interest and the belief that one can find happiness and fulfillment by the use of reason alone. Christian faith is necessary because it shows the way out of sin and the way to spiritual fulfillment. The tension between the freedom to direct the course of history and the inability to overcome finite human nature can be relieved only by surrendering to a complete faith in God’s wisdom, as it revealed in the New Testament and embodied in the figure of Christ.
A life lived by Christian faith balances the needs of the individual and the needs of others; it creates a harmony with nature, with the self, and with others. The sin of inequality and the taint of conscience are also eliminated. One gains eternal life by accepting God’s twofold grace, that of redemption from sin and that of resurrection through spiritual elevation. A belief in the redemptive power of God’s love frees one from the anxiety caused by knowing that one will die and the feeling that life has no purpose or meaning beyond the present. Christian faith frees one from the guilt of sinfulness because of Christ’s redeeming act of self-sacrifice, and Christian faith offers eternal life.