Sheed & Ward
In this essay, the theological response to the question is an ecclesiological one: there are degrees of membership in the Church, from the explicitness of baptism to a nonofficial or "anonymous" form of Christianity. Rahner arrives at this position by starting with what can be said of the human being as spirit, that is, unlimited openness to the being of God, a capacity for God, an innate tendency toward God. In turn, the Incarnation, where God enters into human reality and becomes human being, reveals the fundamental ordering of God toward the human person (a subject in an interpersonal, intersubjective historical situation). Explicit Christian faith, which is expressed within the interpersonal communion of the Church, expresses what is always and already an implicit experience among human beings qua human. So, if a human being who does not explicitly profess Christian faith nevertheless says yes to her life in relation to the horizon of grace that we call God, then that person could be called a Christian, albeit anonymously, even though that person is not an explicit member of the Church. Such was the state of the question when Rahner was looking at it in the late 1950s and mid -1960s: a matter of ecclesiology, membership in the Church through the radical acceptance of God's grace in a human life in a mode that is intrinsically Christian.
Crowley, P. G. (2005). Introduction: Improbable Encounters? In P.G. Crowley (Ed.) Rahner beyond Rahner: A Twentieth Century Theological Giant Meets the Pacific Rim. Rowman and Littlefield/Sheed & Ward, (p. xiii-xx).