When conservative Paul IV became pope in 1555, he instituted a rigorous campaign to suppress heresy in the Italian States. Aconcio felt himself threatened and in June 1557 he renounced his Catholic faith and fled first to Basel and then on to Zurich. In Basel he met Bernardino Ochino and other radical Italian reformers. In Switzerland he wrote his first works, "Dialogue di Silvio e Mutio", outlining the Lutheran criticisms of the Catholic church and "Summa de Christiana religione", which presents a view of religion purged of the controversial points that divided Christendom. Both were published in 1558. At the same time he published a secular work, "De Methodo, hoc est, de recte investigandarum tradendarumque Scientiarum ratione", which lays out an approach to intellectual inquiry that emphasizes a rational, almost mathematical approach, proceeding from clear and concrete first principles.