This week's Torah reading begins: "Pinchas, the son of Elazar... turned My wrath away from the children of Israel by zealously taking up My cause among them.... Therefore... I have granted him a covenant of peace. He and his descendants will possess an eternal covenant of priesthood, because he zealously acted on behalf of his G-d."
Our Sages identify Pinchas with the prophet Elijah. Elijah's Divine service was also characterized by zealousness, as it is written: "I have been very zealous for the sake of G-d, the L-rd of Hosts." When making this statement, however, Elijah contrasted his own conduct with that of the Jewish people at large, whom he criticized for "forsaking [G-d's] covenant."
G-d refused to accept these words of criticism. He appointed Elijah as "the angel of the covenant" and charged him with attending the circumcisions of the Jewish people for all time to come, so that he would attest to our people's faithful adherence to G-d's covenant.
With this, G-d was teaching Elijah that his zealousness must be tempered with ahavas Yisrael, love for every member of our people, and that he must endeavor to seek out our people's virtues. These traits became such an integral part of Elijah's personal mission that when the prophet Malachi describes Elijah's return to announce the coming of the Redemption, he states that Elijah will "turn the hearts of the fathers to the children, and the hearts of the children to the fathers." Thus when the Rambam describes Elijah's mission, he states that "he will come solely to establish peace." To emphasize this direction from the outset, G-d tells Pinchas that in reward for his zealousness, he is being given "a covenant of peace." For without that modulating influence, much of the positive value of zealousness will be lost.