Christianity was first introduced into Silesia by missionaries from Moravia and Bohemia. After the conversion of Duke Mieszko I of Poland and the conquest of Silesia, the work of bringing the people to the new faith went on more rapidly. Up to about the year 1000 Silesia had no bishop of its own, but was united with neighbouring dioceses. In this way, the connection of Silesia with the Holy Roman Empire continued. The upper part of the Oder River formed the boundary of the Kingdom of Poland. All the territory which is now Silesia – lying on the right-hand bank of the Oder – belonged, therefore, to the Diocese of Poznań, which was suffragan to the Archbishopric of Magdeburg. This part of Silesia was thus under the jurisdiction of a priest named Jordan who was appointed first Bishop of Poznań in 968. The part of Silesia lying on the left bank of the Oder belonged to the territory included in then Bohemia, and was consequently within the diocesan jurisdiction of Prague. The Bishopric of Prague, founded in 973, was suffragan to the Archbishopric of Mainz.
Duke Bolesław I the Brave, the son of Mieszko, obtained the Bohemian part of Silesia during his wars of conquest, and a change in the ecclesiastical dependence of the province followed. By a patent of Emperor Otto III in 995, Silesia was attached to the Bishopric of Meissen, which, like Poznań, was suffragan to the Archbishopric of Magdeburg. Soon after this emperor Otto and Bolesław, who had pledged allegiance to the emperor, then ruled the entirety of Silesia, founded the Diocese of Wrocław, which, together with the Bishoprics of Kraków and Kołobrzeg, was placed under the Archbishopric of Gniezno in Greater Poland, founded by Otto in 1000 during the Congress of Gniezno. The first Bishop of Wrocław is said to have been named Johannes, but nothing more than this is known of him, nor is there extant any official document giving the boundaries of the diocese at the time of its erection. However, they are defined in the Bulls of approval and protection issued by Pope Adrian IV, 23 April 1155, and by Pope Innocent IV, 9 August 1245.
The powerful Polish ruler Bolesław I was succeeded by his son Mieszko II Lambert, who had but a short reign. After his death a revolt against Christianity and the reigning family broke out, the new Church organization of Poland disappeared from view, and the names of the Bishops of Wrocław for the next half century are unknown. Casimir I, the son of Mieszko, and his mother were driven out of the country, but through German aid they returned, and the affairs of the Church were brought into better order. A Bishop of Wrocław from probably 1051 to 1062 was Hieronymus, said by later tradition to have been a Roman nobleman. He was followed by Johannes I (1062–72), who was succeeded by Petrus I (1071–1111). During the episcopate of Petrus, Count Peter Wlast entered upon that work of founding churches and monasteries which has preserved his name. Petrus was followed by: Żyrosław I (1112–20); Heimo (1120–26), who welcomed Otto of Bamberg to Wrocław in May, 1124, when the saint was on his missionary journey to Pomerania; Robert I (1127–42), who was Bishop of Kraków; Robert II (1142–46); and Johannes II (1146–49), who became Archbishop of Gniezno.
With the episcopate of Bishop Walter (1149–69) the history of the diocese of Wrocław begins to grow clearer. Pope Adrian IV, at Walter's request in 1155, took the bishopric under his protection and confirmed to it the territorial possessions of which a list had been submitted to him. Among the rights which the Pope then confirmed was that of jurisdiction over the lands belonging to the castle of Otmuchów (Ottmachau) which had been regarded as the patrimony of the diocese from its foundation. In 1163 the sons of the exiled Polish duke Władysław returned from the Empire and, through the intervention of Emperor Frederick Barbarossa, received as an independent duchy the part of Silesia which was included at that date in the see of Wrocław. Bishop Walter built a new, massively constructed cathedral, in which he was buried. Żyrosław II (1170–98) encouraged the founding of the Cistercian monastery of Lebus by Duke Bolesław I the Tall. In 1180 Żyrosław took part in the national assembly at Łęczyca at which laws for the protection of the Church and its property were promulgated. Jarosław (1198–1201), the oldest son of Duke Bolesław, and Duke of Oppeln, was the first prince to become Bishop of Wrocław (see prince-bishop).
Cyprian (1201–7) was originally Abbot of the Premonstratensian monastery of St. Vincent near Wrocław, then Bishop of Lebus, and afterwards Bishop of Wrocław. During Cyprian's episcopate Duke Heinrich I and his wife, St. Hedwig, founded the Cistercian convent at Trebnitz. The episcopate of Bishop Lorenz (1207–32) was marked by his efforts to bring colonies of Germans into the church territories, to effect the cultivation of waste lands. This introduction of German settlers by the bishop was in accordance with the example set by Heinrich I and St. Hedwig. The monasteries of the Augustinian Canons, Premonstratensians and Cistercians took an active part in carrying out the schemes of the rulers by placing great numbers of Germans, especially Thuringians and Franconians, on the large estates that had been granted them. One of the most noted bishops of the diocese, Thomas I (1232–68), continued the work of German colonization with so much energy that even the marauding incursions of the Mongols (1241) made but a temporary break in the process. As German colonization in Silesia increased the city of Wrocław began to be known by the German name of Breslau, leading to the diocese also becoming called the Bishopric of Breslau. Thomas's defence of the rights of the Church involved him in bitter conflicts with Duke Bolesław of Liegnitz. Thomas began the construction of the present cathedral, the chancel being the first part erected. St. Hedwig died during his episcopate; and he lived until the process of her canonization was completed, but died before the final solemnity of her elevation to the altars of the Catholic Church. After Thomas I, Ladislaus, a grandson of Saint Hedwig, and Archbishop of Salzburg, was Administrator of the Diocese of Breslau until his death in 1270.
He was followed by Thomas II (1270–92), who was involved for years in a violent dispute with Duke Henry IV as to the prerogatives of the Church in Silesia. In 1287 a reconciliation was effected between them at Regensburg, and in 1288 the duke founded the collegiate church of the Holy Cross at Breslau. Before his death, on the Eve of St. John in 1290, the duke confirmed the rights of the Church to sovereignty over the territories of Neisse and Ottmachau. Thomas II consecrated the high altar of the cathedral; he was present at the First Council of Lyon (1274) and in 1279 held a diocesan synod. Johann III Romka (1292–1301), belonged to the Polish party in the cathedral chapter. His maintenance of the prerogatives of the Church brought him, also, into conflict with the temporal rulers of Silesia; in 1296 he called a synod for the defence of these rights.
In the election of Heinrich I of Würben (1302–19), the German party in the cathedral chapter won, but this victory cost the new bishop the enmity of the opposing faction. He was made guardian of the youthful Dukes of Breslau, and this appointment, together with the factional disputes, led to the bringing of grave accusations against him. The researches of more recent times have proved the groundlessness of these attacks. He was kept in Avignon a number of years by a suit before the Curia which was finally settled in his favour. Notwithstanding the troubles of his life he was energetic in the performance of his duties. He carried on the construction of the cathedral, and in 1305 and 1316 held diocesan synods. The office of Auxiliary Bishop of Breslau dates from his episcopate. After his death a divided vote led to a vacancy of the see. The two candidates, Weit and Lutold, elected by the opposing factions, finally resigned, and Pope John XXII transferred Nanker of Kraków to Breslau (1326–41).