Brenan resolved to make public reparation for his defection to Anglicanism.
On the Sunday in 1809 that he was announced to commence his campaign against the Catholic Church, he ascended the pulpit of St. George's, began by blessing himself most reverently, and then to the relief of his audience took up the Bible, and said "This is the Word of God".
After a brief pause, he added deliberately and earnestly, "And I swear by its contents that every word I have uttered against the Catholic Church is a lie", and at once left the building.
Brenan went to a neighbouring Capuchin friary, explained what had happened, and begged to be admitted into the order.
After some time, his request was granted, and he became a Capuchin friar at Wexford, where in later years he wrote (as a penance, it is said) his Ecclesiastical History of Ireland (2 vols., Dublin, 1840, revised ed., 1864).