8 He did not hesitate to descend from the heights of wisdom to the basics of writing. He undertook the instruction of children and even composed elementary writing books and grammars for their use. Indeed just as he often came back from preaching to the courts of kings to address the people, so, after learned writings on dogma or morals, he used to compose pamphlets destined either to strengthen the faith of the people or to arouse and nourish their piety. He had wonderful success in preventing the inexperienced from getting caught in the nets of error. The Summa which he published for this purpose is a compact and tightly-knit work, written in beautiful Latin and not unworthy of the Fathers of the Church. This remarkable work was enthusiastically received by learned men in almost all the countries of Europe. Less voluminous but no less useful were the two famous catechisms which this blessed man wrote for less cultivated minds: one for the religious instruction of children, the other for young men already involved in the study of the arts. These two works had such a great success among Catholics immediately upon publication that almost all professors charged with teaching the basics of the faith had them in hand. They were used not only in the schools as a spiritual milk for the children, but they were also explained publicly in the churches to the benefit of all. Thus, during three centuries Canisius has been regarded as the teacher of Catholics in Germany. In popular speech "knowing Canisius" was synonymous with "preserving the Christian faith."
Source: Militantis Ecclesiae (Vatican.va)