In some respects Foundations of Christian Faith is a catechism outlining the basic tenets of faith for all believers. The theology on which Rahner bases his investigations is largely that developed over the centuries by the Catholic Church, although a number of prominent Protestant scholars have embraced much of his work and acknowledged Rahner as one of the premier theologians of the twentieth century. The questions he poses in this book are indeed fundamental to all Christians: How do humans know God? What does it mean to have a relationship to God? What is the nature of Jesus Christ, and what is Jesus’ role in salvation? What is the role of the Church in humankind’s salvation?
In answering these questions, Rahner offers an explanation of the nature of God and especially of Christ. His chapter on Jesus is an extensive treatise on Christology that gives believers a sound grounding for their acceptance of Christ as both human and divine. What Rahner also does with exceptional care and skill is explain the nature of grace, a concept much debated for centuries among Christians. Rahner defines grace as the self-communication God makes with humans, positing it as integral to human nature rather than portraying it as some added bonus that humans may earn or receive.
In keeping with the doctrines of the Catholic Church, of which he is a member, Rahner insists that individuals participate in a communal effort to participate in God’s process of salvation. He does not go so far as to say that salvation is not possible except for those who belong to the Church, but he is insistent that the Church—by which he means the Catholic Church—has a definite role to play in salvation history.