He entered St Chad's College, Durham University having intended to become an Anglican priest, but changed his views in 1962 while completing his undergraduate degree in theology.
[4] His PhD was awarded in 1977 for a doctoral thesis titled "The interpretation of Daniel VII in Jewish and patristic literature and in the New Testament: an approach to the Son of Man problem".
[7][9] Casey's work argued strongly for Aramaic sources behind the New Testament documents, specifically for Q and the Gospel of Mark.
He also contributed works on early Christology and the use of the term Son of Man within the New Testament Gospels in reference to Jesus.
According to Casey, Jesus would preach, heal people with psychosomatic disorders, and be crucified and buried, but would not be physically resurrected; the story of the empty tomb is, according to his views, a legend.