To get the First Church of Springfield, the only church in Springfield that the Simpsons citizens go to, you must be at least level 14 and have activated the quest called Passion of the Flanders Pt. 1. The First Church of Springfield is the church that is run by none other than Minister Reverend. Lovejoy or Rev. Lovejoy for short just like in The Simpsons: Tapped Out. If you cannot activate the quest then it is more than likely you have not satisfied the previous conditions which may be one or more. Simply go back to your task book and ensure that all the quests are complete and that everyone in your Tapped Out town is not busy. Sooner or later someone will pop up with a speech bubble telling you to complete their quest.
When you finish building the First Church of Springfield you will also unlock and get Rev. Lovejoy to join your town. You will also gain a little more income tax from the First Church of Springfield build that earns you 200 cash and 20 xp every 12 hours. By unlocking the First Church of Springfield you also gain access to a lot more quests from Ned Flanders as well as other characters that are related to studying the bible.