The only reference to Bishop Thomas during his episcopate in Finland is a letter signed by him in Nousiainen in 1234, which granted certain lands around the parish to his chaplain, Wilhelm. The lands may be related to the papal permission from Pope Gregory IX in early 1229 that authorized the church to take over all non-Christian places of worship in Finland. The letter is the first surviving letter ever written in Finland.
No further information on bishop's activities has survived before he was granted resignation by Pope Innocent IV in February 21, 1245. According to the Pope, Thomas had admitted committing several felonies, like torturing a man to death and forging a papal letter. Church representatives to oversee the resignation were the Archbishop of Uppsala and the Dominican prior of the Dacian province. Thomas donated his books to the newly established Dominican convent in Sigtuna and went on to live his last years in the Dominican convent in Visby, Gotland. He died there in 1248, shortly before the Second Swedish Crusade which cemented the Swedish rule in Finland for more than 550 years.
During Thomas' episcopate, Finland is listed among the lands under the papal legate in the Baltic region, originally the Bishop of Zemgale, Baldwin, and then William of Modena, first in January 28, 1232 and last July 15, 1244. This was a radical realignment of the bishopric's position, since the Pope had earlier used Swedish bishops to assist the Finnish church, as evident from papal letters from 1171 (or 1172), 1221 and 1229. It can be suspected that the events in Sweden, with papal favourite King Eric temporarily deposed by the anti-church Knut Långe in 1229, may have played a part. In November 24, 1232, the Pope even asked the Livonian Brothers of the Sword to provide forces for the unnamed Bishop of Finland to defend the country against the Novgorodian attacks.
After Thomas had resigned in 1245, there was no immediate successor to him. The diocese continued to be overseen by William at least until June 5, 1248. Finland is not listed among the Swedish dioceses in surviving documents from 1241 and 1248, but appears among them in 1253.
Even though Thomas is the first known Bishop of Finland, it is certain that he was not the first bishop overall. An unnamed Bishop of Finland is mentioned dead in a letter by Pope Innocent III already in 1209. A 15th century chronicle names bishops Henry, Rodulff and Folquinus before him, but no indisputable records survive of them.
Being the first historical figure of importance in Finland, historians have tried to attribute Thomas with more significance than what is evident from the existing sources.
Most commonly, Thomas is speculated to have been the unnamed Bishop of Finland to whom Pope Gregory IX replied in January, 1229 with several letters of great importance to the church, in the aftermath of major Finnish losses in the battle against the Republic of Novgorod. Church representatives ordered by the Pope to assist the unnamed bishop were the Bishop of Linköping and the Cistercian abbot in Gotland.
Thomas' identification with the bishop remains doubtful, as there are good reasons to assume that Thomas was a Dominican monk and it is unlikely that a Dominican was already a bishop in Finland when they first arrived to Gotland in 1228. It can also be argued that involvement of Cistercians in the 1229 events makes a Dominican bishop questionable. Noteworthy is also that the Pope had given permission to transfer the see to a "more suitable" location in 1229 and that three years later, it seems to have been in Nousiainen in the church of Virgin Mary, the aim of Cistercians' utter devotion.
Cistercians dominated the missionary work in the eastern Baltic Sea until end of 1220s. Soon thereafter, they were thrown into the background, when Teutonic Knights and Pope Gregory IX started to favor Dominicans. Christian of Oliva, a Cistercian strong man in the Baltics, had been imprisoned in 1233 for several years, which quickly paved way for Dominicans to take over.
A surviving letter by Pope Gregory IX directly to the chaplain of Nousiainen in October 20, 1232 makes the Finnish see appear vacant. The letter handled the same land dispute that Thomas himself addressed two years later. In some copies of the letter, the Bishop of Finland is also referred to as "N.", while not directly saying whether he was still in office or not.
A papal letter to an unknown Bishop of Finland, certainly not the same person as Thomas, was also sent already in 1221. For a reason or another, this bishop seems to have been completely removed from the history of the Finnish church during the Swedish era.
Violent anti-church clashes in Tavastia, central Finland, mentioned in a letter by Pope Gregorius IX in 1237 have been attributed to Thomas' harsh methods of Christianization, however without direct evidence of that. Oddly enough, the letter, addressed to the Archbishop of Uppsala, does not mention Bishop or Diocese of Finland in any way. Information about the uprising had also originated from the temporarily sidelined Archbishop, who seems to have used the occasion to remind the Pope about Uppsala's earlier contributions to the missionary work in the north. The Pope had clearly not known where Tavastia exactly was, and eloquently urged the Archbishop to send in a crusade.
Noteworthy is also, that the Livonian Brothers of the Sword had been all but annihilated in the Battle of Saule in 1236. Even if there is no other evidence on their presence in Finland than the earlier papal letter from 1232, both the Archbishop and the Tavastians seem to have been on the move right after their demise. There had also been a revolt against the Germans in Estonia in 1236.
Based on the letter, some historians have tried to date the so-called Second Swedish Crusade to 1238 or 1239, listing it as Thomas' accomplishments as well.
Thomas is also speculated to have been one of the driving forces behind the Battle of the Neva, a disputed Swedish-Novgorodian conflict that took place in 1240. The speculation is based on the Russian Primary Chronicle that mentions Finns and Tavastians to have fought on the Swedish side, which according to some historians would have been organized by the bishop. However, as the Chronicle also lists the very unlikely Norwegians as Swedes' allies, the information is often regarded as the mid-14th century propaganda, the time when the Chronicle was written, with Sweden in control of Norway, Finland and Tavastia.