Thomas Erastus, a Swiss physician and theological polemic, who exchanged his original name of Lieber for its Greek equivalent, born in Baden, Sept. 7,1524, died in Basel, Dec. 31, 1583. He studied theology and literature in Basel and medicine in Bologna, practised his profession with success, and after being for many years professor of physic at Heidelberg, obtained in 1580 the chair of ethics at Basel. A skilful practitioner, relying on induction from experience rather than on dogmas and theories, he was a formidable opponent of the fancies of Paracelsus and his disciples. His principal theological controversies were in the conferences of Lutheran and reformed divines at Heidelberg and Maulbronn on the Lord's supper, in which he maintained that the reference to the body and blood is figurative. He also had a controversy with Dathenus and Beza concerning the doctrine of excommunication. Ho held that ecclesiastical censures should extend only to divergences in theological opinion, and not at all to vices and immorality, which were civil offences, and properly punishable only by temporal magistrates.

In some of his writings he seems to favor the principle that all ecclesiastical authority is subordinate to the civil power, which is the doctrine commonly recognized as Erastianism. His treatise Be Coena Domini (1565) was translated by Shute (London, 1578); his theses on excommunication, written in 1568, were first published in 1589 by Castelvetro, who married his widow. An English translation of his theses was published in 1669, and was reedited by the Rev. Robert Lee (London, 1845).