This is the book by WC Smith that has inspired me now for many years.
This book is difficult to obtain. I have two second-hand copies, one to keep for myself and one to lend out to anyone interested.
The following essay has material related to the topic, though it’s not complete, not a full exposition of what Smith was thinking of:
WC Smith – Comparative Religion (12k words)
Here is a YouTube lecture on Tillich and WC Smith:
The bit on Smith starts 3 mins in.
There is a nasty “verdict” on Smith’s world theology in the form of a photo of a drunken Obama with the cynical caption “Good luck with that plan buddy”. The photo comes up at 9:07 and fades out at 9:11, a rather inauspicious moment?
It’s also curious that Obama was chosen here. He offered hope to many people that he could heal the world, being essentially both a Muslim (by birth and early upbringing) and a Christian (by conversion) as well as being the son of a black man and a white woman. A seemingly perfect coincidentia oppositorum (or union of opposites). And really, it did not work out so well.
Update, 8 Feb 2019:
I have written and self-published a booklet titled Offering a New World Theology. It aims to be a simple, readable, and colourful response to Smith’s call.
Kindle edition: https://www.amazon.com.au/dp/B07L9J4D7J/