The Council of Nicaea takes an important moment with enormous impact on society and plonks the Doctor, Peri and Erimem right into the middle of it. While the modern cliche is to have the Doctor trying to stop an alien menace from changing history, author Caroline Symcox rightly avoids this and lets history speak for itself. And ultimately that was the right choice.
The Council, held way back in 325 CE, was finally called to discuss the differences in the Christian church, particular interest was held with questions about the divinity of Jesus and the relationship between Jesus and God. The decisions that the Council made would become the foundations of the Christian orthodoxy and as the Doctor points out, the Nicaean Creed is still spoken in churches today. But despite this setting, the listener shouldn't fear about being subjected to a two-hour sermon. Instead the story focuses on big aspects of social tension, propaganda, justice and the law and lets each member of the TARDIS team to pick their own side on the events.
The Doctor seems to uncomfortably take the side of the law. It is his job to make sure that history remains on course without changing a fixed point in time. Because of this he ends up the first prisoner and then ally of the power at the time - the Emperor Constantine. He will judge the results of the council and he is more interested in stopping the riots than he is on determining theology.
Peri is placed in an even more uncomfortable position. She has no fixed beliefs and is comfortably used to multiple churches from her own time and life in the USA. At first, she can't even take the Council seriously. But what she does take seriously is being used as a pawn by the bishops and Constantine and how deeply it hurts her friend, Erimem.
And then there is Erimem who feels more deeply about all of this. Nicaea isn't too far removed from her own time of Ancient Egypt. The first person she meets is the heretic Arius and his treatment from the Council and the mobs outrages her real sense of justice. To her frame of mind, this isn't something that happened a long, long time ago and if she has anything to say about this, it isn't going to be a fixed point in time. She meddles with events to the point where the Doctor is forced to threaten stranding her. But as far as Erimem is concerned, that would be a brilliant outcome.
But she isn't the only one to consider the results of the Council as a life or death matter. Some of the other faction leaders have already killed in the name of their cause and won't hesitate to do it again if these strangers continue their interference.
But at the end of the day, despite the fights and riots, The Council of Nicaea isn't really an action story. It is a real character piece, allowing the listeners to see how these individuals react to and grow as a result of exploring a really pivotal moment in history...