Barlow told his friend Sir Peter Pett that the real ground of hostility was not his avoidance of Lincoln, but his continuing hostility to Catholicism. In 1678, when Titus Oates forwarded his theory of a Popish Plot, Barlow had publicly declared enmity to the papists and their supposed leader, James, Duke of York. When the bill enforcing a test against popery was introduced, which excluded such peers from the House of Lords, Gunning of Ely defended the church of Rome from the charge of idolatry, but Barlow answered him vehemently. In 1680, while the Popish Plot panic was still at its height, he republished under the title of "Brutum Fulmen", the papal bulls of Pius V and Paul III, pronouncing the excommunication and deposition of Queen Elizabeth and of Henry VIII, with inflammatory comments, and learned proofs that "the pope is the great Antichrist, the man of sin, and the son of perdition." In 1682 appeared Barlow's answer to "whether the Turk or pope be the greater Antichrist." He confirmed his answer in a letter of 1684 to the Earl of Anglesey, arguing again that "the pope is Antichrist."