He rejected the doctrine of the priesthood of believers, he defended the hierarchical order of the Middle Ages and the primacy of the pope, though on these very points his differences with the representatives of the papal system were apparent.

Gropper took a zealous part in the negotiations for church union and in the religious colloquies held in 1540 and 1541 in Hagenau, Worms, and Regensburg.

When the victory was won, and the previous coadjutor, Adolf von Schaumburg, had been enthroned as bishop, Gropper received the provostship in Bonn formerly held by a brother of Hermann.

Under the new archbishop, Gropper worked by word and pen against the Protestants; he likewise acted as imperial commissioner for the carrying out of the Augsburg Interim in his native city of Soest.

The pope received him with honor and demanded his opinion in weighty matters; nevertheless, not only did he not accomplish that which he wished in the Cologne affair, but he was denounced to the Inquisition by the Venetian Delfino.

The pope, probably convinced by Gropper's defense that he was innocent, spoke before a consistory on March 15 in praise of the services of the deceased and transferred his benefices to his brother Kaspar.