This is a difficult read. Unless you are familiar with religious doctrines from the late 1700s, there's not much by ease of explanation. Furthermore, Priestly changed his stance on religious matters frequently in life. He treats his chemistry experiments as a short and somewhat trivial jog into the natural sciences. If you want to know more about his views, I recommend Letters to a Philosophical Unbeliever. There, one will find as I did, at the end a response by a Mr. William Hammond, which this book reveals to be the disguised pseudonym of a Mr. Turner. I believe Turner's retort at the end of Letters is among the most poignant and as yet relevant answer an atheist can give in the face of persecution. Preistly's thoughts on this are either lost, or, unlikely, he has no words. Find it in Project Gutenberg and read it.