13 Under such evil circumstances therefore, each one is bound in conscience to watch over himself, taking all means possible to preserve the faith inviolate in the depths of his soul, avoiding all risks, and arming himself on all occasions, especially against the various specious sophisms rife among non-believers. In order to safeguard this virtue of faith in its integrity, We declare it to be very profitable and consistent with the requirements of the time, that each one, according to the measure of his capacity and intelligence, should make a deep study of Christian doctrine, and imbue his mind with as perfect a knowledge as may be of those matters that are interwoven with religion and lie within the range of reason. And as it is necessary that faith should not only abide untarnished in the soul, but should grow with ever painstaking increase, the suppliant and humble entreaty of the apostles ought constantly to be addressed to God: "Increase our faith."(11)
Source: Sapientiae Christianae (Vatican.va)