Irenæus 
Against Heresies [EPUB ebook] 

When the Christian church was first forming in the time of the apostles, a heresy that still threatens the church today called Gnosticism started to form. This heresy was battled by the writings of Irenæus to correct theological thinking about God, Jesus, and the Bible at large. This early church father was instrumental in fighting false teaching in the early centuries when there were a lot of questions about the legitimacy of Christianity in the face of the persecutions of the Roman Empire.