[3] The bishop of Namur, Nicolas-Joseph Dehesselle, appointed him to establish the first Catholic normal school in modern Belgium, at Malonne.

[3] On 27 September 1852, Pope Pius IX confirmed Montpellier's appointment as bishop of Liège in succession to his recently deceased friend, Cornelius van Bommel.

As bishop, Montpellier encouraged the foundation of hospices, hospitals and orphanages; the building of new parish churches (consecrating 88 in person); and the establishment in his diocese of many religious orders and congregations, especially those running schools.

[3] He frequently preached Advent and Lenten missions in the major churches of his diocese, and led annual retreats for his clergy.

[3] He actively opposed Liberal legislation to suppress religious processions and funerals, reallocate the income of Catholic parishes and schools, and subject seminarians and clergymen to military conscription.