Lesslie Newbigin was (and is, through his writings) a celebrated missiologist. While on the surface of it, this book gives the impression of being an eclectic mix of ideas, there is a fundamental cohesion to the book. In fact it represents a fundamentally new approach to missiology.
The book may arguably be said to have one underlying theme: epistemology, or the theory of knowledge. That is, how can we know? How can we have confidence in the gospel "in the midst of a plurality of cultures and religions"? Newbigin, in his own words, has "relied heavily on the work of Michael Polanyi." Polanyi's epic work "Personal Knowledge" was published nearly fifty years ago, and reveals what might be said to be a coherence theory of truth. That is, if one's beliefs should cohere as a whole, this should be a good indication of truth. Polanyi, however, adds a radical twist to this. He writes about "the coherence of commitment". That is, once one has formed a responsible opinion about "truth", one needs to commit to it passionately, and publish. Only in this way can one both display integrity, and submit one's "truth" to the scrutiny of others -- to be affirmed, modified, or perhaps even overturned. It is not hard to see how this relates to missiology. In terms of this view, the gospel requires commitment and proclamation. This in turn leads to a confirmation of its truth in various ways -- or it may lead to a revision of Christian beliefs and practices.
Newbigin further applies Polanyi's epistemology to virtually every aspect of Christianity. He undertakes a broad task of synthesis, or reconciliation, within the Church. He suggests "a third way of understanding Christian belief" -- a method which seeks to take Scripture on its own terms, and which (he hopes) would be acceptable to Christians of virtually every persuasion. This represents, arguably, much of the drawing power of Newbigin's ideas.
However, Newbigin's epistemology is not without its problems. Not least, Polanyi himself considered that there would be "absurdly remote chances" of successfully applying his philosophy to Christianity, and that even a witch doctor "may gain a limited justification within a society" (p. 318, Second Impression 1962). Further, it seems doubtful that Newbigin gives adequate account of how a living God might find a place within an (apparently) closed theory of truth.
All having been said, Newbigin is intellectually agile, he writes with conviction, and his ideas have a considerable reach. He also shares many interesting insights gained in missions over nearly forty years, as well as important observations on the Church in the West.