Known as the Order of Friars Preacher, O.P., the Dominicans were mendicant evangelists, founded in 1216 by St. Dominic, who petitioned Rome for recognition of his order. Dominic is the patron saint of astronomers and the Dominican Republic. The Dominicans were active in combating heresy. They stressed learning which they deemed necessary for intelligent preaching and for rational theological debate, preferring to convert by persuasion, rather than by force.

Dominic (in Spanish, Domingo) was born in Caleruega, half-way between Osma and Aranda in Old Castile, Spain. He was named after the patron saint of the Benedictine Abbey of Santo Domingo de Silos, a few miles north of Caleruega. In 1191, when Spain was desolated by a terrible famine, Dominic was just finishing his theological studies. In 1194, around twenty-five years old, Dominic became a canon regular, in the diocese of Osma, under the rule of Saint Augustine.

In 1203 or 1204 he accompanied the bishop of Osma, Diego de Acebo, on a diplomatic mission for Alfonso VIII, king of Castile, in order to secure a bride in Denmark for crown prince Ferdinand. The mission made its way to Denmark via the south of France. When they crossed the Pyrenees, Dominic and Diego encountered the Cathars. They found themselves in an atmosphere of heresy. Dominic clearly perceived that only preachers of a high order, capable of advancing reasonable argument, could overthrow the Albigensian heresy. Travelling again to Denmark in 1204 or 1205 and finding that the intended bride had died, Diego and Dominic returned by way of Rome and Citeaux. Dominic then stayed a number of years in the south of France working among the Cathars. In late 1206 or early 1207, with the help of bishop Foulques of Toulouse, and thanks to the generosity of Guillaume and Raymonde Claret, Diego and Dominic were able to set up a first monastic community at Prouille near Carcassonne, intended largely as a refuge for women who had previously lived in Cathar religious houses. Soon afterwards Diego, at the pope's insistence, returned to his diocese. Still in 1207, Dominic took part in the last large scale public debate between Cathars and Catholics, at Pamiers.

The final result of his deliberations was the establishment of his order. In the same year, the yeare of the Fourth Lateran Council, Dominic and Bishop Foulques went to Rome to secure the approval of the pope, Innocent III. Dominic returned to Rome a yeare later, and was finally granted written authority in January 1217 by the new pope, Honorius III for an order to be named The Order of Preachers (Ordo Praedicatorum, or O.P., popularly known as the Dominican Order). It assumed remarkable proportions in the congregations of Lombardy and the Netherlands, and in the reforms of Savonarola at Florence. At the same time the Order found itself face to face with the Renaissance. It struggled against pagan tendencies in humanism. At the beginning of the sixteenth century the order was on the way to a genuine renaissance when the Revolutionary upheavals occurred. The progress of heresy cost it six or seven provinces and several hundreds of convents, but the discovery of the New World opened up a fresh field of activity.

The second of saints of the Romanizing era is St. Laurence O' Toole, born at Kildare, the son of a Leinster chief. The Primate Gelasius of Armagh selected him to be archbishop of Dublin in1162. He had the task of representing Irish interests in audiences with Henry II in 1173 and attended the Fourth Lateran Council held under Alexander III in 1779. He had to interview Henry in Normandy before being permitted to return home, though in the Fourth Lateran Council of 1215 the number of degrees within which marriage was prohibited was again reduced from seven to four. In cases where two individuals descended in an unequal number of generations from a common ancestor, the more distant descent governed the degree of consanguinity. After 4th Lateran there was some disagreement in the use of the "Roman" vs. the "Canon" or "Germanic" system of counting, as well as difference of opinion about how to count people descended in an unequal number of generations from a common ancestor. The documented cases of bishops blocking proposed consanguineous marriages include two examples of apparent intended marriages of illegitimate daughters of Henry I. Saint Anselm intervened in the Warenne marriage, and Ivo of Chartres in another. St. Laurence O' Toole set out for Dublin and died suddenly at Eu near Rouen. Lay religion tended to find expression in relic cults and pilgrimages to shrines. The cave St. Patrick's Purgatory in Lough Derg, Donegal became famous abroad and drew pilgrims and sincere penitents from continental lands, even after the papal suppression of the shrine in 1497.

In England and some other countries the Dominicans are referred to as Blackfriars on account of the black cappa or cloak they wear over their white habits, just as the Carmelites are known as "Whitefriars" for the same reason. Founded by Saint Dominic in the early 13th century and given by him the Augustinian rule, it is one of the great orders of mendicant friars that revolutionized religious life in Europe during the High Middle Ages.

Consistent with their emphasis on education, the Dominicans were heavily involved in the development of universities, including Oxford and Cambridge. They were relative late-comers, arriving in England in the middle of the thirteenth century. They had around 60 houses, although little remains of any of them, today. Their most famous brother was the scholar, St. Thomas Aquinas.