Firmicus Maternus, De errore profanum religionum, is a very interesting late Roman text on paganism from the mid-4th century. Unfortunately it has never been online.
A correspondent kindly lets me know that a PDF containing a 1971 thesis with a full translation can be found here (PDF here): Richard E. Oster, Julius Firmicus Maternus: De errore profanum religionum. Introduction, translation and commentary, Rice University, 1971. I suspect that this is the same person who blogs here, with many of the same interests (and outlook) as myself.
Grab it while it’s hot! Rice University are to be commended for making this accessible.