Thank you for the question on the most read philosopher and theologian, “Who is the most read philosopher and theologian?”
I don’t know if someone can say definitely who is the most read philosopher and theologian.
I would rank Bertrand Russell as one of the the top philosopher and atheistic theologian if I may use that term. The source is:
https://people.ucalgary.ca/~rzach/blog/2009/03/whos-most-famous-philosopher-of-20th-century.html
Some may consider Soren Kierkegaard as the greatest philosopher and theologian but he may not be the most widely read philosopher. Based on popularity, Bart Ehrman, with his books being on the Best Seller’s list, will be among the most widely read atheistic writer. That comment is to state his theological belief, not meant to demean him or his writings.
C.S. Lewis of the past generation would rank high among Christians, but he is no longer widely read and even less Thomas Aquinas and St. Augustine.
Combine philosophy and theology is not an easy task. The rise of atheists and theists debates has become more center stage in the last 75 years.
SUMMARY: Bertrand Russell is probably the most widely read philosopher and theologian.