Translating the Message can be seen as a long historical reflection on Pentacost and its aftermath. The essence of how the Gospel relates to cultures lies in its "translatability," Sanneh argues. Jesus spoke in Aramaic, but the records of his life are in Greek. At each stage of translation, missionaries tend to demand that hearers learn their own "civilized" ways along with the Gospel. But the nature of that message mitigates against this cultural presumption, so that when the Gospel has been translated, indigenous people find biblical support for their own independence from the missionaries and their (not infrequently imperialistic) culture.
As Sanneh argues, while human beings may share foibles, in this respect Christianity contrasts sharply with Islam. The Koran was written in heaven in pure Arabic. Sanneh's point here is right on the mark: an Iranian friend of mine converted to Christianity partly because, when he went to Mecca, he was told that since his Arabic was so poor, he should divorce his wife! I'm not sure of the logic, but the upshot was that he recognized Arab culture as sautered to Islam. V. S. Naipaul's books describe how this works in places like Iran, Pakistan, Malaysia, and Indonesia. More and more, as education spreads and people want to read their sacred books directly, Muslims around the world are asked to become Arab.
Sometimes Christians have made the same mistake. What Sanneh explains well here, is that despite our stupidity, the Gospel itself, both its content and the very fact that it is translated, eventually encourages a plurality of Christian cultures to spring up.
If this pluralism matters so much, one reviewer asked, why has Islam also won the allegience of so many Africans? The 20% of Africans living north of the Sahara were converted in the initial military expanse of Islam. 25% or so of Africans live south of the Sahara, and converted over 1000+ years of military and economic expansion from that powerful northern base. Another 40% live south of the Sahara and have converted to Christianity, mostly in the past 150 years, mostly voluntarily. I think that pattern does show that affirming cultures is better than denying them, even as strategy: otherwise with its huge head start, one might have expected Islam to have easily swept through the rest of Africa.
Quite a bit of the book deals with Africa, though Sanneh also talks about Greece, Europe, India, the Americas, and Japan a fair amount. Translating the Message can be read as a universal history of Christianity, from one particular perspective.
My area of expertise is China and East Asia. My first book 15 years ago was called True Son of Heaven: How Jesus fulfills the Chinese Culture, so I've been thinking about these issues for a while. Sanneh doesn't talk about East Asia much, here, but what he says is interesting. A few characters who are especially fascinating in the light of his thesis, whom he either fails to mention, or says little of: the Nestorian missionary Jing Jing; Mateo Ricci (of course); the Jesuit missionary to Vietnam, Alexander Rhodes (an amazing and successful career, following Ricci's example, but great at friendship and at trusting Vietnamese); James Legge (the China translator par excellence); and John Ross, who read Legge and can almost be seen as the founder of the Korean church, though he lived in China. For those who are interested in my part of the world, I recommend you look into the lives of these remarkable men.
My approach to Gospel and world cultures is through what I call "Fulfillment Theology," which is more involved than translation, but Sanneh lightly touches on some of these deeper issues here, too.
Sanneh is writing for an educated audience that is willing to invest time and thought into following his argument. He tends to repeat himself a fair amount, so if you skip a few pages, you won't miss his point, though you might miss a good example, or even a good story, some amusing. Some passages are a little top-heavy with abstract nouns, others flow smoothly, or achieve a sort of eloquence. Anyone can learn a lot from this book.