Answer:
When Winston sees an engraving of St Clement Danes in Charrington's shop he recognizes the place but realizes he didn’t know that it was once a church. They have a conversation about how difficult it was to know history because the Party´s propaganda would claim any impressive buildings as having been built since the Revolution, deeming anything from an earlier time as part of a dark period vaguely called the Middle Ages.
Step-by-step explanation:
Even architecture has been made to reflect the Party´s version of history and religion. Nevertheless, the engraving of the church gives Winston a reminder of the past, which becomes an obsession to him because it provides truthful evidence of history as opposed to the doctrine of the Party which claims having saved humanity from capitalist oppression and misery.