Below you will find a PDF of the first letter written by Saint Theophilus of Antioch to his pagan friend, Autolychus. It is one of three surviving letters and is the one we reference in our Patristix episode about the word 'Christian'. The letters aren't easy to read to be honest, they're quite complex. But they present a wonderful image of Early Christian apologetics. Theophilus works through his knowledge of pagan philosophy and stories, to try and bring his friend to the One True Faith. If you have an interest in Greek mythology, early church writings, and just learning more, you can find all three letters at this website.
Otherwise, here is the first letter.