Christianity in the Reach derives mostly or wholly from texts. Any missionary activity is so far back and obscure that it is legendary. Some Old Settler religions appear to have Christian antecedants, but none of them are or consider themselves Christian. It is a minority religion, though a large one (about 10% of the population).
Their minority position tends to keep them united despite a wide variety of doctrinal opinion. Most planets have several congregations, each headed by a bishop and assisted by deacons; each planet usually has an archbishop, elected by the bishops; the archbishop, in turn, appoints bishops. The archbishops belong to interplanetary "colleges"; a given archbishop may belong to more than one. These colleges are more for communication than for the exercise of authority. The Empire, each of its provinces, and the whole Reach have colleges.
Besides all this, there are a large number of freelance Christians unaffiliated with any congregation or moving freely from one to another. There are also hermits, small monastic groups, and friars (deacons with no fixed congregation but recognized by one or more bishops).
Most Imperial Space Christian congregations observe five sacraments: (infant) baptism, confirmation, communion (with both elements), marriage (permitted to monastics and clergy), and ordination (of deacons and bishops). They use the Protestant Canon but often believe in purgatory. They use the Apostles' Creed.
The main body of Christian teaching arrived in the Reach in a single package called the kerygma ("the teaching"). The first Christians in the Reach were either self-converted by reading the Kerygma they discovered or perhaps converted by missionaries who brought the Kerygma. However, no records have been found of these missionaries and they remain legendary.
The Kerygma arrived at the time of the New Terraformers; all sources agree on this. It may have come in the libraries on the New Terraformer ships (with or without human missionaries) or it may have been transmitted to the Reach over one of the rare and seldom-used telesis lines to the Ecumene.
The legendary missionary and bringer of the Kerygma is St. Iris Kerygmaphora. She is very likely eponymous; her last name just means "teaching-bearer," and her first name is suspicious since it is the same as the mythological messenger of the Greek gods. However, there is no reason there could not have been a real missionary named Iris.
Wherever the Kerygma goes, there one also finds St. Evangelus Simulacrum. Unlike St. Iris, he is real, but he is not human; he is an interface characterization, software capable of assisting the user of the Kerygma database. Legend provides a fleshly St. Evangelus (or sometimes an AI), companion to St. Iris and the orignal of the characterization, but his reality is even more dubious than that of St. Iris.
The Kerygma database contains: