The church was a place of worship for the populace in London, the Priest residing there was either a religious fanatic (judging by the writing on the wall) or simply went insane during the chaos of the Original Outbreak.
When Jim woke up in an abandoned hospital 28 days after the initial infection, he set about exploring the streets of London, the first place he entered was an apparently empty church. Jim procceeded to explore the main hall and came across a decomposing pile of dead bodies that seemed to indicate a mass suicide of some kind.
Jim called out to see if anyone was alive and attracted a couple of infected that were laying amongst the corpses, asleep. He then hears the Infected Priest bashing on the other side of the locked door in the hall. When the priest smashes the door open and tries to attack Jim, the latter hits him with the bag of soda cans, Jim then escapes with the earlier infected in pursuit.