Augustine of Hippo — as excerpted in the Catena Aurea on Matthew 5:31-32

“(Retract. i. 19. 6.) Yet I would not have the reader think this disputation of ours sufficient in a matter so arduous; for not every sin is spiritual fornication, nor does God destroy every sinner, for He hears His saints daily crying to Him, Forgive us our debts; but every man who goes a whoring and forsakes Him, him He destroys. Whether this be the fornication for which divorce is allowed is a most knotty question—for it is no question at all that it is allowed for the fornication by earnal sin.”