15 The Church knows very well that "the imagination and thought of man's heart are prone to evil." Therefore she wages continual battle against vice and error "in order that the body of sin may be destroyed, that we may no longer be slaves to sin." Since she is her own mistress and is guided by the grace which "is poured forth in our hearts by the Holy Spirit," she is directed in this conflict in thought and action by the Doctor of the Gentiles, who says, "Be renewed in the spirit of your mind...And be not conformed to this world, but be transformed in the newness of your mind, that you may discern what is the good and acceptable and perfect will of God." The true son of the Church and reformer never thinks he has attained his goal. Rather, with the Apostle, he acknowledges that he is only striving for it: "Forgetting what is behind, I strain forward to what is before, I press on towards the goal, to the prize of God's heavenly call in Christ Jesus."
Source: Editae Saepe (Vatican.va)