In the entire 5th chapter of Matthew, our Lord is calling His disciples to be perfect in order to break our pride and to humble (poor in spirit v.3) us and urge us to call on our Lord for mercy. But at the same time He calls on us to live by divine standards, out of thanksgiving for our salvation. In our attempt to do so we are daily reminded of our sinfulness and we will grow in our appreciation for the Lord's mercy.
Jerome, against the Pelagians, says: "We are righteous when we confess that we are sinners, and that our righteous-ness stands not in our own merit, but in God's mercy." Therefore, when starting to fulfill the Law, faith ought to be present, which certainly believes that we have a reconciled God for Christ's sake. For mercy cannot be received except through faith. We ought to understand: People regenerated through faith not only receive the Holy Spirit, and have motives that agree with God's Law, but we ought also to realize that they are far distant from the Law's perfection. This point has the greatest importance by far, and we must add it to the argument also. We cannot conclude that we are counted righteous before God because of our fulfilling of the Law. Justification must be sought elsewhere in order that the conscience may become peaceful. For we are not righteous before God as long as we flee from God's judgment and are angry with God. We must conclude that we are counted righteous for Christ's sake being reconciled through faith. This is not because of the Law or our works. Christ's death and satisfaction ought to be placed far above our purity, far above the Law itself. This truth ought to be set before us so that we can be sure of this: We have a gracious God because of Christ's satisfaction and not because of our fulfilling the Law. (paragraphs 52-57)
Condensed from CONCORDIA: THE LUTHERAN CONFESSIONS, copyright 2005,2006 by Concordia Publishing House. Used by permission. All rights reserved. To purchase a copy of CONCORDIA, call 800-325-3040.