Most Finns are converted to the Roman Catholic Church by about the mid-twelfth century, during the wave of crusades that began in 1095.

A quasi-historical legend maintains that in 1157 a crusade was led against the polytheistic Finns by the Swedish King Erik IX and the English monk Henry, who had been appointed archbishop of Uppsala.

According to tradition, Henry was martyred in Finland and was subsequently recognized as the country's patron saint.

The success of the crusade was supposed to have given Sweden and Latin Christianity a solid foothold in Finland.

There is no evidence of the crusade and Henry's role in it, however, and there are indications that Christian communities existed in Finland at an earlier date.