Barrientos was appointed an Inquisitor during this time, and in 1438 was elected Bishop of Segovia. On 3 May 1440 a church council was convened in the Church of San Miguel of Turégano, at which he presented his synodal commandments for the theological and pastoral formation of his clergymen. Being loyal to John II, in 1442 he reformed the diocese of Segovia, which had been under the influence of the rebellious prince Henry, to serve the province of Ávila in supporting the King. Later, he engineered a conciliation in the Tordesillas area to create a joint front in the face of the opposition of the Kingdoms of Navarre and Aragon. In 1444 he occupied the diocese of Cuenca and in the following year he lent the diocese's aid to the region's troops in the First Battle of Olmedo. Similarly, in 1449, he pledged the arms of the diocese in defense against the troops of don Alonso de Aragón and other Castilian noblemen who opposed the king. One offered him the archbishopric seat of Santiago de Compostela, but he rejected this enticement.