Subtle and clever, he was well fitted for a diplomatic career, and carried out several missions in England (1527-1534) and Rome (1534-1536). In 1535 he received his cardinal's hat; in 1536-1537 he was nominated "lieutenant-general" to the king at Paris and in the Tie de France, and was entrusted with the organization of the defence against the imperialists. When Guillaume du Bellay went to Piedmont, Jean was put in charge of the negotiations with the German Protestants, principally through the humanist Johann Sturm and the historian Johann Sleidan.