The present volume consists of the Conrad Grebel Lectures for 1960, delivered at Eastern Mennonite College, Goshen College, and Hesston College. I am grateful to the Lectureship Committee which set the theme and gave the commission to write on a topic of major importance as well as one currently the object of much intense interest not only in my own denomination but in Christendom at large. I am grateful also to the committee for its sugges¬tions during the preparation of the lectures, and for the helpful criticism and reactions received from friends who heard or read the lectures, in particular my colleagues in the Associated Mennonite Biblical Seminaries. I am great¬ly indebted, too, to the many books and articles which have appeared in recent years on the nature of the church and its mission. Specific acknowledgment to certain authors and publishers is made elsewhere in the following pages. I am particularly indebted to my wife Elizabeth and my daughter Mary for their critical reading of the manuscript, from which I have greatly profited.