https://www.logos.com/product/52218/christian-philosophy-in-the-early-church
It's not that expensive to begin with, and inexpensive with the monthly sale.
I'm always a bit shy on Christian history books, since they tend to back-filter with more modern dogma or views. This one tends to be pretty clean … and sticking pretty close to what can be demonstrated. This one looks at the interplay between philosophy (largely greek), and the development of Christian thinking.
And if you're not comfortable with that angle, take a pass.