[Matthew] 5:43-48 The new righteousness exemplified by the law of love. The Law enjoined love for the neighbor; legalism (the attempt to find favor with God and to stand in His judgement by way of works of the Law) raised the question, "Who is my neighbor?" (Lk 10:29) and sought to limit the imperative of love by finding scope for lovelessness and hate. Jesus removes every limitation from the law of love by enjoining love for the enemy. The highest "righteousness" is love. Jesus went this way of love for before His disciples and for them; He joined Himself, in love, to men under the wrath of God when He was baptized; He loved the enemy in order to "fulfill all righteousness" (3:15). That way of love took Him to the cross, "so that in Him we might become the righteousness of God." (2 Co 5:21)"
Martin Franzmann, Concordia Self-study Commentary [commentary on Matthew], p. 20