In 1527 he was in Paris as a student; he adhered to Lutheranism and was forced to flee Paris, returning to Orbe. The Bernese, lords of the region, welcomed Guillaume Farel, who entrusted Viret with preaching in his native town; he delivered his first sermon on 6 May 1531, not yet twenty years old. He violently attacked Catholics at Orbe, then at Grandson and Payerne, and finally at Geneva, first with Farel and later with Calvin. He then carried on religious activity at Lausanne for 23 years and took part in the founding of the city’s Protestant Academy. Deprived of his office by Bern on account of the dispute over excommunication, in 1559 he moved to Geneva, but for reasons of health he had to emigrate to southern France. He exercised notable influence in the region of Nîmes and Tarascon, where he drew many Catholics—laymen, priests, and religious—to Calvinism. He is found at Lyon in 1562, at Orange in 1565, and finally at Orthez in Béarn, in the service of Jeanne d’Albret. On 5 May 1875 a monument was erected to him at Orbe.
Numerous works by him are preserved in the libraries of Bern, Lausanne, Neuchâtel, and Geneva. The principal one is: Instruction chrétienne en la doctrine de la Loi et de l'Evangile (Geneva 1564), in the form of a dialogue.