I understand it to refer to those who strive to be just, despite our imperfection in being just. It might be similar to our being called to be perfect, as our Father in Heaven is perfect. Obviously we will never be perfect (while on earth), but we can strive for it.
Ver. 17. For the justice of God. He does not here mean that justice, by which God is just in himself, but that justice, or sanctification, which he communicates to men, and by which they are justified and sanctified. — From faith to faith. That is, by faith, and an increase in faith, inasmuch as, by increasing in faith, we advance in virtues; as it is written, (Habacuc ii. 4.) the just man liveth by faith; including the love of God, hope, and other virtues. (Witham)