What triggered Lockwood's nightmare?

This is a Chapter Three question. Lockwood's nightmares were triggered by reading a sermon by Reverend Jabez Branderham.

I began to nod drowsily over the dim page: my eye wandered from manuscript to print. I saw a red ornamented title - 'Seventy Times Seven, and the First of the Seventy-First.' A Pious Discourse delivered by the Reverend Jabez Branderham, in the Chapel of Gimmerden Sough.'