We organize our history according to key dates, which seem to mark sharp boundaries in the flow of events. The year 1865 is an obvious American example, and 1945 plays a similar role for much of the world. These are the moments when crucial historical trends self-evidently ended or began, and we naturally choose them to frame our books or our television documentaries. When, as so often happens, history refuses to be so precisely framed, we glide over the inconsistencies. Neatness is all.

In the history of Christianity, and of the civilization built upon it, the year AD 325 represents one such natural division. May 2025 marks the 1,700th anniversary of what we usually think of as one of the most decisive moments in that history, namely the great council that the emperor Constantine called at Nicaea, near his new city of Constantinople. Of the pivotal importance of the event there seems little doubt. We speak of the church’s earliest days in terms of Ante-Nicene Christianity, or the age of the Ante-Nicene Fathers. Something, surely, must have changed at that point. But exactly what that might have been remains a matter of intense debate.

The story of Nicaea is quickly told. In 312, Constantine consolidated power in the Roman Empire and granted toleration to Christianity the following year. He accepted some leading Christians as his advisors on religious matters and felt the need to demonstrate his leadership of the larger church when it fell into crisis or division.