At the service on Friday, on the eve of the Feast of the Immaculate Conception, Dr Williams encouraged his hearers to keep working on ecumenical relations.
"When we're tempted to talk of an ‘ecumenical winter', tempted to focus our attention only on the stresses and conflicts between communions of Christian Churches, it's worth remembering that our God is a God who keeps his promise and that he has promised to be with us."
Dr Williams' final London engagement as archbishop - before he becomes Master of Magdalene College, Cambridge, in the New Year - will be a dinner with the papal nuncio to Britain, Archbishop Antonio Mennini, at his residence in Wimbledon.