David Yezzi at The New Criterion:
A second spur for Auden was his experience during the Spanish Civil War, where he witnessed the destruction of churches and the persecution and murder of the clergy. As Carpenter writes, “In all, several thousand clergy members of religious orders fell victim to Republican persecutors, and this was only a fraction of the total number of people murdered on the Republican side.” Auden was deeply disillusioned by what he found in Spain. He later said that he “could not escape acknowledging that, however I had consciously ignored and rejected the Church for sixteen years, the existence of churches and what went on in them had all the time been very important to me.”
Indeed, churchgoing again became very important to him. At one point, when Auden was living on Middagh Street in Brooklyn, Golo Mann notes, “On Sundays, he began to disappear for a couple of hours and returned with a look of happiness on his face. After a few weeks he confided in me the object of these mysterious excursions: the Episcopalian Church.”
more here.