I noticed that sometimes these two are used interchangeably, especially as designations of older, Medieval written documents. I know that 'epistle' is a letter, whereas 'treatise' is a written work concerned with a specific subject; surely, they are not synonymous. But can they ever be mutually inclusive? Any information regarding these two words is appreciable.
In layman's terms an epistle is a 'letter'; whereas a treatise is essentially an 'argument in writing'. (https://www.merriam-webster.com/dictionary/epistle) (https://www.merriam-webster.com/dictionary/treatise)