Having been ordained in 1775, he for a short time held a cure at Misslitz, but was soon recalled to Bruck as professor of Oriental languages and Biblical hermeneutics.
In 1792 he published his Einleitung ins Alte Testament (2 volumes), which soon brought him into trouble; the cardinal-archbishop of Vienna laid a complaint against him for having departed from the traditional teaching of the Church, e.g. by asserting Job, Jonah, Tobit and Judith to be didactic poems, and the cases of demoniacal possession in the New Testament to be cases of dangerous disease.
An ecclesiastical commission reported that the views themselves were not necessarily heretical, but that Jahn had erred in showing too little consideration for the views of German Catholic theologians in coming into conflict with his bishop, and in raising difficult problems by which the unlearned might be led astray.
[1] Although he appears honestly to have accepted this judgment, the hostility of his opponents did not cease until at last (1806) he was compelled to accept a canonry at St Stephens, Vienna, which involved the resignation of his chair.
The only work of importance, outside the region of mere philology, afterwards published by him, was the Enchiridion Hermeneuticae (1812).