From our perspective today, the Reformation does seem like an obvious outgrowth of the Northern Renaissance. Luther came to his beliefs through close study of the Bible, which was something the Northern/Christian humanists brought about. He believed in the idea of people reading the Bible for themselves in the vernacular, which was another Northern humanist idea. He followed Erasmus's idea of reforming the Church to return it to what it had been like nearer to the time of Christ. His ideas were spread by the universities that had become more important in the Northern Renaissance.
However, this did not seem so obvious to everyone at the time. Many of the original humanists, including Erasmus, did not support the split that Luther caused. Even so, most of the younger humanists did support the split and the Reformation does seem to us today like an obvious outgrowth of the Northern Renaissance.