The Christian church was badly disrupted by the invasions of the 800s and early 900s as well as by the rise of the local strongmen that accompanied the invasions. In Normandy five successive bishops of Coutances resided at Rouen, far from their war-torn district, which had converted to paganism under the Vikings. Elsewhere standards of clerical deportment declined, threatening the moral leadership with which Carolingian prelates had supported public order. Renewal came in two influential forms.

First, monks in Burgundy and Lorraine were independently inspired to return to a strict observance of the Benedictine rule and thereby to win the adherence of laypeople anxious to be saved. The monastery of Cluny, one centre of reform, was founded in 910 by William I (the Pious), a duke of Aquitaine with a bad conscience; dedicated to Saints Peter and Paul, it thus came under the protection of the pope. The Cluniac reform, whose influence gradually radiated beyond Cluny and encouraged reforms in other monastic houses, stressed independence from lay control, opposed simony and clerical marriage, and practiced an elaborate routine of liturgical prayer. In the 11th century Cluny came to direct an order of affiliated monasteries that extended throughout France and beyond. Cluny’s religious hegemony was challenged only in the 12th century with the rise of a yet more ascetic Benedictine observance, of which St. Bernard of Clairvaux (1090–1153) was the great proponent. Centred at Cîteaux (Latin Cistercium, whence the appellation Cistercian) in Burgundy, this movement combined ascetic severity with introspective spirituality and economic self-sufficiency. A newly personal devotionalism was diffused from monastic cloisters into lay society.

Second, the bishops, in the absence of royal leadership, renewed Carolingian sanctions against violence. The Peace of God was instituted in synods of southern France in the late 10th and early 11th centuries. Solemnized in relic processions and oaths and supported by large crowds of the laity, it was an effort to restrain the increasing number of knights from violating the traditional rights of peasants and churches. It was supplemented from the 1020s by the Truce of God, which forbade fighting on certain days or during particular seasons of the year and which helped to mold a new conception of the knight as a Christian warrior prohibited from shedding the blood of other Christians. These movements were warmly embraced by the Cluniac pope Urban II when he preached the First Crusade at the Council of Clermont in 1095, which resembled the Peace councils earlier in the century. The ideals and reforms of the Peace and Truce of God contributed to a new understanding of knighthood as an honourable estate of Christian leadership. When young princes were dubbed to knighthood in the 12th century, they assumed a mode of respectability fashioned by the church; this eased the way for lesser knights to be recognized as nobles as well.

Scholars such as Gerbert of Aurillac, the future pope Sylvester II, were forced to wander from city to city in the pursuit of learning (Gerbert had to travel to Spain to study advanced mathematics); nevertheless, the growing wealth and stability of regional societies, such as those in Burgundy, Flanders, and Normandy, encouraged new impulses in the arts and letters. Cathedral schools revived the traditional curriculum of learning, stressing reading, writing, speaking, and computation. Fulbert of Chartres (c. 960–1028) was fondly remembered as a humane teacher by students who often became teachers themselves. A century later, famous masters could be found at Laon and Paris as well as (probably) at Chartres, attracting young clerics to their lectures in swelling numbers. The Breton Peter Abelard (1079–1142) taught and wrote so brilliantly on logic, faith, and ethics that he established Paris’s reputation for academic excellence. His famous correspondence with his beloved Héloïse reveals the emerging humanism in 12th-century letters, demonstrating a knowledge of Classical authors and depth of emotion characteristic of the age. Traditional pursuits of contemplative theology and history gave way to new interests in logic and law. Men trained in canon and Roman law found their way increasingly into the service of kings, princes, and bishops.

Everywhere churches were built in Romanesque style, and they continued to be built in the south long after some architects, such as Suger at Saint-Denis in the 1140s, introduced the new aesthetic of Gothic style, a distinctive French innovation. Lay culture found expression in vernacular epics, such as La Chanson de Roland (The Song of Roland) in Old French, and in the Provençal lyrics of southern France. These poems are witness to diverse zones of linguistic evolution from spoken Latin; by the 12th century the langue d’oïl (Old French) north of the Loire was broadly differentiated from the langue d’oc (Occitan, or Provençal, language) to the south. The cultural cleavage so marked ran deeper than language and was not entirely overcome by the spread of modern French, descended from the langue d’oïl, into the south.

At the same time that society and the church underwent reform and expansion, they also faced the first expressions of popular heresy since late antiquity. In the early 11th century, episodes of heresy occurred in Aquitaine, Arras, Orléans, and Vertus. The heretics, possibly influenced by foreign missionaries and certainly reacting against the abuses of the church and failures of reform, rejected the church and its sacraments, abstained from sexual intercourse and eating meat, and lived pious lives. By the mid-11th century the church had successfully repressed the heretics, burning a dozen or so at Orléans under order of the king. Heresy disappeared until the early 12th century, when a number of heretical leaders, such as Peter of Bruys and Henry of Lausanne, developed large followings in various cities. These leaders, again reacting to the flaws of the church and inadequacies of reform, rejected the church, its ministers, and its sacraments and advocated lives of simple piety in imitation of the Apostles.