The effect of Sacred Scripture is twofold: it teaches man to know the truth and persuades him to work justice. Jn 14:26: But the Paraclete, the Holy Ghost...will teach you all things to be known, and bring all things to be done to your mind. And therefore it is profitable to know the truth and direct it into action.
For there is the speculative reason and the practical reason. And in each two things are necessary: that it know the truth and refute errors. For this work is the work of the wise man, namely, not to lie and to refute the liar. Regarding the first he says, is profitable to teach, namely, the truth. Ps 118:66: Teach me goodness and discipline and knowledge. Regarding the second he adds, to reprove. Tit. 1:9: That he may be able to exhort in sound doctrine and to convince the gainsayers.
[Thomas Aquinas, Commentaries on St. Paul's Epistles to Timothy, Titus, and Philemon, Baer, tr. St. Augustine's Press (South Bend, IN: 2007) p. 137. From the commentary on II Timothy.]