The substance of this little volume was given as lectures in seminaries in the U.S. and in Europe by the retiring General Secretary of the World Council of Churches. Its theme is concerned with the temptation to syncretism on the part of Christianity in its attempts to encounter and to spread itself in the face of other world religions. Four waves of syncretism are identified and described with considerable historical knowledge and insight. Against these are author insists upon the central affirmation of Christianity, the revelation of Himself, once for all, of God in Jesus Christ. He is less successful in suggesting an attitude and a program for creative encounter on the part of Christianity with other religions. We need, he says, a new humility. But the impression remains that he regards other religions outside the realm of God's revelation or activity, and so leaves unanswered the question as to what Christianity proposes to do about them. A valuable little book for its historical survey of the problem more than for its future pointing proposals.