How do we know the difference between essential and nonessential doctrines or beliefs?

Dirk Jongkind, co-leader of the Theologians Network, is a Dutch biblical scholar who finished his PhD at Cambridge University. His main scholarly interest is in the Greek text of the Bible and the Graeco-Roman backdrop of Acts and the letters. Currently, he is the Research Fellow in New Testament Text and Language at Tyndale House, Deputy Senior Tutor at St Edmund's College, Cambridge, and affiliated lecturer at Cambridge University. He has done much work on Greek manuscripts and other remains from the ancient world.