A necessary effect of love, being the vehement movement of one who loves to [secure] the object of his love
Zeal (from delos, a derivative of deo) "to boil", to "throb with heat"), is "a necessary effect of love", being "the vehement move-ment of one who loves to [secure] the object of his love" (vehemens motus amantis in rem amatam, St. Thomas, "Summa Theol.", I-II, Q. xxviii, a. 4). Here the distinctive note is in the vehemence, or intensity, of the action to which love impels, an intensity which is proportioned to that of the love felt. As there are two kinds of love, the amor concupiscentioe, which is self-regarding, and the amor amicitiae, which is altruistic, two corresponding kinds of zeal might be distinguished, but by usage the term is restricted to the zeal prompted by the amor amicitioe; indeed, in its religious sense it is applied solely to the zeal inspired by the love of God, to the ardent endeavors and works undertaken to promote His glory. Here again we can subdivide according as this zeal for God manifests itself in works of devotedness directed towards the fulfilment of the first or the second of the two great Commandments. In the Bible (cf. Psalm lxiii, 10; Num., xxv, 11; Tit., ii, 14, etc.) it is mostly used in the first of these applications; in the phrase "zeal for souls" it is used in the second, and in this sense it is much the most common among religious writers.
Zeal, being love in action, just on that account tends to remove as far as lies in its power all that is injurious or hostile to the object of its love; it has thus its antipathies as well as its attractions. Moreover, since, though itself appertaining to the will, it presupposes an exercise of judgment as to the appropriate means for the attainment of its object, we must further distinguish true and false zeal, according as the judgment guiding it is sound or unsound. Thus St. Paul's zeal was zeal through-out, but it was false zeal in the days when he persecuted the Church, true zeal when he became its Apostle. "Caritas Christi urget nos" are the words with which this Apostle described the promptings within his own breast of this zeal which contributed so powerfully to lay the foundations of the Catholic Church. And it is a zeal of like nature which, enkindled in the breasts of so many generations of ardent followers of Christ, has, in its cooperation with the lavish gifts of the Holy Spirit, built that Church up into the greatest marvel of human history. For it is the zeal of all those devout souls which, as distinguished from the lukewarmness of the ordinary Christian, has sent forth the Apostles and missionaries to their lives of self-sacrifice, has filled the sanctuaries with an unfailing supply of good priests and the cloisters with throngs of fervent religious, which has organized, sustained, and developed so splendid an array of works of charity to meet almost every conceivable need of suffering humanity.
SYDNEY F. SMITH