"All Christ's faithful are obliged by divine law, each in his own way, to do penance...The days and times of penance for the universal Church are each Friday of the whole year and the season of Lent" (Can. 1249-1251).
The Imitation of Christ says: "What does it avail you to argue profoundly of the Trinity if you be wanting in humility, and consequently be displeasing to the Trinity? I would rather feel compunction, than to know its definition" (Book I, Ch. 1/3).