In recent decades, the image of Hilaire de Poitiers before his exile in 356 has often been that of a provincial bishop, still uninvolved in the doctrinal quarrels of his time. Jean Doignon’s important monograph, Hilaire avant l’exil, has helped to develop this image. How could a bishop who, by his own admission, knew nothing of the Nicene symbol before being exiled to Phrygia, have known and fought against the Arians (or those who were considered Arians at the time)? This view is based in particular on an analysis of the commentary that the Bishop of Poitiers dedicated to the Gospel according to Matthew (In Matthaeum), which a unanimous consensus dates from before his exile. The present study challenges this now-dominant portrait, taking a fresh look at the commentary’s anti-heretical polemic. Combining the results of an analysis of the work’s main Trinitarian and Christological developments with an exploration of its references to heretics and heresy, it provides new evidence in favor of Hilary’s early commitment to countering the offensive led from 353 onwards by Constantius II and some prominent bishops to impose non-Nicene dogmatic formulas. It even turns out that, although rarely noticed until now, this context had a profound influence on certain pages of the commentary, where Hilary slips in discreet allusions to this imperial campaign, encouraging his fellow-bishops to resistance. (Forthcoming [June 2024])