Well, you’re going to need to get them familiar with the Early Church Fathers. First, discuss with them the “alternate” forms of Christianity out there, specifically, Gnosticism, Docetism, and Arianism, and describe to them what precisely they believed. Especially with the first two, you need to show that most of their sources are written well after those that are codified into the Bible. Next, you need to show how their beliefs are radically different than anything around today. These were not Protestant groups, but groups with beliefs more akin to the Scientologists. You then need to quote Early Church Fathers that demonstrate that many regions were concerned about these heretical groups and their radically wrong beliefs, and also show how the structure of Bishops was clearly around very early on. This crisis of alternative beliefs finally leads the church to decide on the “minimum” requirements to be considered Christian (The Nicene Creed developed at the Council of Nicaea). They also decide to codify the Bible, to make it clear that there are very valid reasons why other books are NOT included. (You’ll want to investigate those reasons, btw.) You also want to demonstrate how thoroughly organized the church was BEFORE Constantine, so that they know that he wasn’t the one coming up with all of this.