"The concept of "righteousness of God" was explained in the Letter to Titus: "But when the kindness and generous love of God our savior appeared, not because of any righteous deeds we had done but because of his mercy" (Titus 3:4-5). Saying "The righteousness of God appeared," is the same as saying: The goodness of God, his love and his mercy appeared. It was not man who, all of a sudden, changed life and tradition and put himself to the task of doing good; the novelty is that God acted, he was the first to extend his hand out to sinful man, and his action fulfilled time.
Here is the novelty that distinguishes the Christian religion from any other. Any other religion draws out for man a path to salvation by means of practical observations and intellectual speculations, promising him, as a final prize, salvation and illumination, but leaving him substantially alone in achieving the task. Christianity does not begin with what man must do to save himself, but rather with what God has done to save him. The order is reversed.
It is true that to love God with all your heart is "the first and greatest of the commandments," but the commandments are not primary, they are secondary. Before the order of commandments comes the order of gift and of grace. Christianity is the religion of grace! If this is not taken into consideration in interreligious dialogue, the dialogue would be able to do no more than generate confusion and doubts in the hearts of many Christians.
Everything, then, depends on faith. But we know that there are different types of faith: There is the faith-acquiescence of the intellect, the faith-confidence, the faith-stability, as Isaiah calls it (7:9). What type of faith is addressed when talking about justification "by faith?" It addresses a special type of faith: the faith-appropriation. It does not tire me to cite in this respect a text of St. Bernard:
"But as for me, whatever is lacking in my own resources I appropriate for myself from the heart of the Lord, which overflows with mercy. My merit therefore is the mercy of the Lord. Surely I am not devoid of merit so long as he is not of mercy. And if the Lord abounds in mercy, I too must abound in merits (Psalm 119:156). But would this be my own righteousness? Lord, I will be mindful of your righteousness only. For that is also mine, since God has made you my righteousness." (St. Bernard of Clairvaux, Sermon on the Song of Songs)"
- Fr. Cantalamessa (http://www.zenit.org/article-14845?l=english)