Zelus, quocumque modo sumatur, ex intensione amoris provenit, quia quanto aliqua virtus intensius tendit in aliquid, fortius etiam repellit omne contrarium vel repugnans.
Zeal, whatever way we take it, arises from the intensity of love, because the more intensely a power tends to anything, the more vigorously it withstands opposition or resistance.
Amor autem amicitiae quaerit bonum amici; unde quando est intensus, facit hominem moveri contra omne illud quod repugnat bono amici. Et secundum hoc, aliquis dicitur zelare pro amico, quando, si qua dicuntur vel fiunt contra bonum amici, homo repellere studet.
Love of friendship seeks the friend's good; wherefore, when it is intense, it causes a man to be moved against everything that opposes the friend's good. In this respect, a man is said to be zealous on behalf of his friend, when he makes a point of repelling whatever may be said or done against the friend's good.
Et per hunc etiam modum aliquis dicitur zelare pro Deo, quando ea quae sunt contra honorem vel voluntatem Dei, repellere secundum posse conatur.
In this way, too, a man is said to be zealous on God's behalf, when he endeavors, to the best of his means, to repel whatever is contrary to the honor or will of God.