Epistle etymology: from Latin epistola “a letter,” from Greek epistole “message, letter, command, commission,” whether verbal or in writing.
Apostle etymology: from Late Latin apostolus, from Greek apostolos “messenger, envoy,” literally “person sent forth.” Compare epistle.
I also use the Catholic Encyclopedia for a variety of topics because it has been referenced by popes, which makes it credible. https://www.newadvent.org/
Note: I edit/shorten often because this is a blog. I scan what others write because time is short. For me, writing is a way of processing thoughts. Frequent returns to blog posts undo errors and straighten meanderings unedited by anyone other than myself. Send your comments. I use many resources, such as https://mailchi.mp/thomasaquinas/may23-e-letter?e=81108d0817