A celebrated exorcist; b. 22 Aug., 1727, at Braz, Vorarlberg, Austria; d. 4 April, 1779, at Pondorf, on the Danube (Diocese of Ratisbon); studied at Prague and Innsbruck; ordained priest, 1750, and after serving various missions, became parish priest and dean of Pondorf May, 1776. A few years after his appointment to Klosterle in the Diocese of Chur, Switzerland (1758), his health began to fall, so that he was scarcely able to fulfil the duties of his ministry; he consulted various physicians in vain; suddenly he conceived the idea that his infirmities might be due to the influence of the evil spirit and might be cured by spiritual means. His experiment was successful. He applied this method also to others and soon thousands came to him to be healed. The fame of these cures spread far and wide; he was invited to the Diocese of Constance, to Ellwangen, Ratisbon, and other places; everywhere he had the same success.
He was convinced that the evil spirit could harm the body as well as the soul; and hence that some infirmities were not the result of natural agencies, but were caused by the Devil. Only cases of the latter kind were taken up; he applied the exorcisms of the Church, and commanded the evil one to depart from the afflicted, in the name of the Lord Jesus. To find out whether the disease was caused naturally or not, he applied the "probative exorcism", i.e. he commanded the spirit to indicate by some sign has presence in the body. And only then he made use of the "expulsive exorcism". His proceedings were not secret; anyone of good standing, Catholic or Protestant, was admitted. People of all classes, nobles, ecclesiastics, physicians, and others often gathered around him to see the marvels they had heard of. Official records were made, competent witnesses testified to the extraordinary happenings. The character of the work made many enemies for him, but also many stanch friends and supporters. One of his bitterest opponents was the rationalistic professor Johannes Semler of Halle. Also the physician Mesmer pretended that the cures were performed by the animal magnetism of his invention, but he was afraid of confronting Gassner. Among his friends were the Calvinistic minister, Lavater of Zurich, and especially Count Fugger, the Prince-Bishhop of Ratisbon.
Official investigations were made by the ecclesiastical authorities; and all were favourable to Gassner, except that they recommended more privacy and decorum. The University of Ingolstadt appointed a commission, and so did the Imperial Government; they ended with the approval of Gassner's procedure. In fact, he never departed from the Church's teaching or instructions concerning exorcism, and always disclaimed the name of wonder-worker. He was an exemplary priest, full of faith and zeal, and altogether unselfish in his works of mercy.