This is known today as "Old North Church", because it's the oldest church in the North End of Boston, but up to 1776 it would have been known by its official name, Christ Church, named after Jesus Christ, a former celebrity carpenter.
This is where, on the night of Paul Revere's ride, two signal lanterns appeared to warn rebels on the opposite shore that the British Regulars were on their way to seize weapons. The signal was Paul Revere's idea, but contrary to popular belief the lamps weren't a signal sent to Revere. They were meant to warn lookouts on the opposite shore in case Revere was captured.
This church was a natural choice for a signal flame: it was the highest steeple in Boston and easily seen from the other side of the river. Even better, the church was Anglican - the congregation was full of wealthy Loyalists. Who would suspect the church sexton (Robert Newman, a friend of Revere's) of sneaking into the church at night to send secret messages?
Well, the British would, actually. They captured Newman a few days later, and questioned him. He put on a terrific show, and managed to convince them he was innocent - and then fled town, which didn't really back up the whole 'innocent' thing very well at all.