In converting a people, the Church had to move them away from their old beliefs and practices and win them over to Christianity. Frequently there was a transfer of pagan rituals and habits, like the dedication of springs that had been worshipped to Christian saints or the transformation of a pagan temple. While this certainly made conversion smoother, it was inherently problematic as well. St. Boniface and others not only had to convert people but essentially keep reminding them of their new faith. Conversion seems to be a continual practice-a mass baptism doesn't work in the long run.
Old habits take generations upon generations to disappear, if they disappear at all.
For those who were born Christian, conversion is still a concern. The temptation of sin and un-Christian activities is constant. Bede is full of references to hell. King Edwin, during his conversion, is advised by a counselor to accept Christianity as it offers a more certain idea of the afterlife. Hell is the swirling winter storm through which the poor, belaguered sparrow flies with only a brief respite in the warm mead-hall of life.