Due to the Battle of Svolder of 999-1000, Sigfrid and his nephews left for Sweden in 1002 where they founded three churches in Västergötland (West Gothland). This area was already being Christianized by the bishop Odinkar Hvite the Elder, who was based in Skara. Thus Sigfrid and his nephews went east to the region of Värend in Småland to continue their missionary efforts, erecting first a cross and then building a wooden [[Church architecture|church]] on the shores of Växjö Lake. Soon, eleven prominent men of the area were soon brought to the Christian faith and [[baptism|baptized]] in a spring near the mountain Ostrabo. Their influence began a steady stream of pilgrims, who flocked to hear the Word of God and gaze upon Sigfrid's silken vestments, gold and silver vessels, and other beautiful objects brought with him from England. King Olof Skötkonung soon became curious and sent a trusted councilor to investigate the matter. According to the councilor, as Sigrid lifted the [[paten]] during the [[Divine Liturgy]], the [[prosphora]] became a young Boy, whom he kissed, at which time the vision disappeared. Astonished by this, the king sent for the holy bishop immediately; but en route, Sigfrid made a detour in Utvängstorp to enlighten and baptize the people there.
In 1008, Sigfrid baptized the royal family in a spring near the village of Husaby, making Olof Skötkonung the first Christian King of Sweden, which marked the beginning of the Christianization of Sweden. Sigfrid next traveled north to re-establish the See of Uppsala that had been founded by St. [[Ansgar]], but the reversion to paganism was too strong in the region and he left in failure. During this time, the English bishop Gotebald was sent from the Danish frontier city of Lund to Scania, where he built its first church and was appointed bishop. In 1014, Olof Skötkonung assisted Sigfrid in the establishment of the Diocese of Husaby (later Skara), which became a suffragan of the Archdiocese of Hamburg-Bremen. The first diocesan bishop, a German named Thurgaut, was consecrated by Sigrid. At this time, the diocese was comprised of "the Goth lands" Västergötland and Östergötland (East Gothland), some of Småland, and also a chunk of other surrounding counties. Sigfrid is said to have consecrated a [[vicar bishop]] for the eastern half of the diocese in the city of Linköping in Östergötland, and King Canute sent an English bishop named Bernhard to fill the see of Scania. By 1028, the English missionary Ulfrid attempted a third re-establishment of Christianity in Uppsala, but was martyred upon
proclaiming [[anathema|anathematizing]] Thor and assaulting his idol in the local Thing.
Entrusting the administration of Växjö to Unaman and his brothers, Sigfrid left to spread Christianity in Denmark. While he was gone, however, a local greedy nobleman Gunnar Gröpe gathered a group of pagans together to kill Sigfrid's brothers and ransack the church in Växjö. When Sigfrid returned from his mission, he found his nephews' heads in a weighed tub that was at the bottom of Lake Helgasjön near the church. The bodies were buried deep inside a forest. According to Sigfrid, the heads still possessed the ability to speak and told him the names of their murderers. Sigfrid glorified God that He allowed Unaman, Sunaman, and Winaman to become [[martyrs]]. Soon Olof Skötkonung heard of the occurrence in Växjö and offered to execute the perpetrators, which Sigfrid refused. The King also offered to extract from them weregild, which the saint also refused. Finally, Sigfrid requested landed property and was gifted the estates known as Hof and Tjuby. The laying of foundation remained extremely difficult for the destitute bishop, but he eventually rebuilt the church and placed inside of it the relics of his holy kin.