 The turning point in Calvin 's fortunes occurred when Michael Servetus , a fugitive from ecclesiastical authorities , appeared in Geneva on 13 August 1553 . Servetus was a Spanish physician and Protestant theologian who boldly criticised the doctrine of the Trinity and paedobaptism ( infant baptism ) . In July 1530 he disputed with Johannes Oecolampadius in Basel and was eventually expelled . He went to Strasbourg where he published a pamphlet against the Trinity . Bucer publicly refuted it and asked Servetus to leave . After returning to Basel , Servetus published Two Books of Dialogues on the Trinity ( Latin : Dialogorum de Trinitate libri duo ) which caused a sensation among Reformers and Catholics alike . The Inquisition in Spain ordered his arrest . 
