Final answer:
Desiderius Erasmus, a classical scholar and leading figure of the Renaissance humanist movement, authored the book 'In Praise of Folly' and played a key role in spreading humanism in northern Europe.
Step-by-step explanation:
The priest who wrote the book In Praise of Folly and was responsible for spreading humanism throughout northern Europe was Desiderius Erasmus. Erasmus was a classical scholar and one of the foremost figures of the Renaissance humanist movement. Although he remained a Catholic throughout his life, his critical engagement with the texts of Christianity and his emphasis on moral philosophy and a more individual connection with piety had a profound influence on the religious life in Europe. By encouraging the study of the classical texts of ancient Greece and Rome alongside Christian teachings, he contributed to a shift in the intellectual climate of his time, which ultimately supported the spirit of the Reformation, despite his more conciliatory religious positions.