A church is a large religious facility that can be constructed in your colonies. They represent the large and specialized houses of worship that replaced the early shrines and meeting houses of the earliest settlements.
Benefits[]
Churches increase cross production, speeding immigration from Europe prior to independence. By itself, a church increases your colony's base production from 1 to 2 crosses. The church can accommodate up to 3 preachers. Each unskilled preacher adds 3 crosses. Each firebrand preacher adds 6. Thus the maximum cross production from a church is 39, requiring 3 firebrand preachers, William Penn, and 100% membership in the Sons of Liberty.
A church also allows your colony to produce missionaries. (Note that they do not produce expert missionaries. Those must be educated at a university or by Jean de Brebeuf after he has joined your Continental Congress.)
Churches can be upgraded to cathedrals once your settlement has reached size 8.
Endgame[]
Once your revolution against your colonists' homeland begins, the clerics at the church no longer have a productive ingame function. One common strategy is to remove the education of any firebrand preachers and arm them with muskets. Doing so just prior to declaring independence provides a greater chance of them upgrading.
Mistakes[]
The manual of the original Sid Meier's Colonization misstated the number of hammers required to construct a church as 52.