The author was writing from a Christian perspective - he believed humans (in many disjoint cultures) somehow innately come to know their need for their saviour - and he attributes it to God putting it in their heart.
One Polynesian culture that was completely separated from the world for centuries even came up with the idea of the Creator coming to earth in human form and rescuing them.
If you're interested and want to read more, the book's called Eternity in Their Hearts: http://www.amazon.com/ETERNITY-THEIR-HEARTS-RICHARDSON-DON/d...
As I said, it is written from a Christian perspective. I don't know of any books that tackle the same issue from a secular/non-religious viewpoint. Often times it's hard to get purely unbiased opinion, I've skimmed through books by Christopher Hawkins / Richard Dawkins and these simply hate on religion. They actually sound a lot more "religious" (in the sense people use it these days) than you'd think. They're often a tirade of rants against Christianity and religion in general with no call to reason or any honest examination of the deep questions at hand. (meaning of life, problem of evil, ...) I've found C.S. Lewis to be much more intellectually stimulating in this regard. Even though he takes a Christian stance, his reasoning/logic quite dispassionate/unbiased compared to what's out there. (at least in Mere Christianity).
I've also found discussion groups useful in this regard. My college and Church had discussion groups led by professors and department chairs (who were Christians) which turned out to be quite helpful/constructive. I know there's organization called Veritas Forum (http://en.wikipedia.org/wiki/Veritas_Forum) which tries to bring such discussions to colleges.
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