Of unknown, or at least unclear origin, he embarked on a religious career in his early years, entering the Franciscan Order, studying in England and at the University of Paris.
He preached and lectured on the Immaculate Conception, and rose to seniority under the patronage of the Avignon Papacy and King Robert II of Scotland, becoming Bishop of Galloway and the only Franciscan to hold a Scottish bishopric.
Thereafter he was a staunch advocate of Avignon Pope Clement VII against the English-backed Urban VI, for whom he engaged in partisan preaching and writing, famously challenging any English bishop to settle the issue by single combat.
On 3 October 1371, following a request from King Charles V of France and Robert II of Scotland, he received papal permission to take the Bachelor of Theology degree at the University of Paris; this is his first appearance in contemporary records.
[3] In his later writings he claimed to have studied in Paris and to have lived among the English for seven years, obtaining a good "understanding of their character".
While present, the recently elected anti-Pope, Clement VII, appointed him papal penitentiary "for the English and Irish languages".
They are to impose silence on him and to put Thomas de Rossy, provided to the bishopric by Clement VII and duly consecrated, into peaceful possession.
[20] Henry le Despenser was probably addressed because he was known for his love of fighting, and had led a short expedition into France during the summer of 1383.
[21] Despite his keenness to fight another bishop, Thomas de Rossy had apparently refused the offer of combat given by an English priest because of the latter's lower status.
[23] The charter was issued at Kirkchrist in Twynholm parish, and was confirmed by the (Avignon) Pope on 18 October when Rossy himself was present at the papal court.
[28] His Quaestio de Conceptione Virginis Immaculatae was a reiteration of some of the arguments for the Immaculate Conception made by Duns Scotus.
It was also a refutation of the scholars who had written against it, including Bernard of Clairvaux, Giles of Rome, Bonaventure O. F. M., Richard Middleton, Facinus de Ast, Robert Cowton O. F. M., Alexander of Hales and Gregory of Rimini.
[30] Rossy's second work, the Tractatus Episcopi Candidae Casae de Regno Scotiae in Facto Schismatis contra Anglicos suos Vicinos was a defence of Clement VII and an attack on the legitimacy of "Bartolomeo of Bari" (i.e. Urban VI).