Despite its stylistic distance from Matthew, Mark and Luke, John was never seriously contested as scripture in the way some other early Christian texts were; its high Christology, identifying Jesus directly with the divine Word, made it central to the doctrinal controversies of the 3rd and 4th centuries, cited heavily on every side of the debates that produced the Nicene Creed. Its prologue is read liturgically at the close of the traditional Latin Mass and, in Orthodox practice, as the Gospel for Easter itself, in Greek, precisely because its high, cosmic opening was judged the fittest possible announcement of the resurrection.
John also supplies the Last Supper discourse that runs uninterrupted across four chapters, 14 through 17, ending in Jesus' prayer "that they may all be one," language that has become a touchstone of the modern ecumenical movement seeking Christian unity across denominations. A minority of early Christian groups read John's language of light, darkness, and secret knowledge in a strongly dualist, Gnostic direction, which briefly made some church leaders wary of the book in the 2nd century, a caution called the Alogi controversy; it dissolved quickly and left no lasting mark on the book's canonical standing.