Friendship with Conrad Mutianus and Ulrich von Hutten led him from being an upholder of Scholasticism to become an enthusiastic partisan of Humanism and a violent opponent of the older learning.
About 1515, he wrote the larger part of the Epistolæ Obscurorum Virorum; the 'letters of obscure men' composed by him are the most violent in character, full of venom and stinging scorn against Scholasticism and monasticism.
While in Bologna he had become acquainted with Martin Luther's writings and actions, learned of the violent stand he had taken and approved it as the beginning of a greatly needed reform of the Church; apparently also he had a share in the anonymous broadsides which appeared in Germany.
In his 1531 Apologia, qua respondetur temeritati calumniatorum non verentium confictis criminibus in populare odium protrahere reverendissimum in Christo patrem et dominum Albertum, Crotus defended Albert of Brandenburg against criticism from Luther and Alexander Crosner, accusing the Reformation of sanctioning immorality and blasphemy.
In a letter dated 1532 to Albert, Duke of Prussia, Crotus made his adherence to Catholicism clear, writing that "with the help of God he intends to remain in communion with the Church and allow all innovations to pass over like a disagreeable smoke".