The Pope always emphasizes mercy. Here he quotes Aquinas as saying that mercy is the greatest of all the virtues as far as external works are concerned.
To continue with the Pope's "Gospel of Joy":
- Saint Thomas Aquinas taught that the Church’s moral teaching has its own "hierarchy", in the virtues and in the acts which proceed from them. What counts above all else is "faith working through love" (Gal 5:6). Works of love directed to one’s neighbor are the most perfect external manifestation of the interior grace of the Spirit: "The foundation of the New Law is in the grace of the Holy Spirit, who is manifested in the faith which works through love". Thomas thus explains that, as far as external works are concerned, mercy is the greatest of all the virtues: "In itself mercy is the greatest of the virtues, since all the others revolve around it and, more than this, it makes up for their deficiencies. This is particular to the superior virtue, and as such it is proper to God to have mercy, through which his omnipotence is manifested to the greatest degree".