ostentation: making a great show.
Puritanism: the more sober style of life and thought introduced by the Puritans, who were a religious party in the times of Elizabeth and the Stuarts, and were desirous of a purer and simpler doctrine and mode of living.
[56. UNDER GEORGE II. PART II.]
Predecessors: those that went before them.
cruciform: in the form of a cross. The ground plan of many churches is shaped like a cross.
St. Stephen's, Walbrook, stands behind the Mansion House, where the Walbrook used to flow.
lectureship: the office of a lecturer, one who gives lectures, discourses, or (as in this case) sermons. Money was left to pay for these sermons, that is, the lectureships were endowed.
harbouring: sheltering.
organisers: those who get up and arrange anything.