The Lord his name is jealous.  God is jealous then, Theotimus, but what is his jealousy? Truly it seems at first to be a jealousy of cupidity such as is that of husbands for their wives: for he will have us so to be his, that he will in no sort have us to be any other's but his. No man, saith he, can serve two masters.  He demands all our heart, all our soul, all our mind, and all our strength; for this very reason he calls himself our spouse, and our souls his spouses; and names all sorts of separations from him, fornication, adultery. And high reason indeed has this great God, all singularly good, to exact most rigorously our whole heart: for ours is a little heart, which cannot supply love enough worthily to love the divine goodness. Is it not therefore meet, that since we cannot give him such measure of love as were requisite, that at least we should love him all we can? The good which is sovereignly lovable, ought it not to be sovereignly loved? Now to love sovereignly, is to love totally.
However, God's jealousy of us is not truly a jealousy of cupidity, but of sovereign friendship: for it is not his interest that we should love him, but ours. Our love is useless to him, but to us a great gain; and if it be agreeable to him, it is because it is profitable to us: for being the sovereign good, he takes pleasure in communicating himself by love, without any kind of profit that can return to him thereby; whence he cries out making his complaint of sinners by way of jealousy: They have forsaken me, the fountain of living water, and have digged to themselves cisterns, broken cisterns, that can hold no water.  Consider a little, Theotimus, I pray you, how delicately this divine lover expresses the nobility and generosity of his jealousy: They have left me, says he, who am the fountain of living water. As if he said: I complain not that they have forsaken me because of any injury their forsaking can cause me, for what the worse is a living spring if men do not draw water at it? Will it therefore cease to run, or to flow out on the earth? But I grieve for their misfortune, that having left me, they have chosen for themselves wells that have no water. And if, by supposition of an impossible thing, they could have met with some other fountain of living water, I would lightly bear their departure from me, since I aim at nothing in their love, but their own good. But to forsake me to perish, to fly from me to fall headlong, is what astonishes and offends me in their folly. It is then for the love of us that he desires we should love him, because we cannot cease to love him without beginning to be lost, and whatever part of our affections we take from him we lose.
Put me, said the divine shepherd to the Sulamitess, as a seal upon thy heart, as a seal upon thy arm.  The Sulamitess had her heart quite full of the heavenly love of her dear lover, who, though he possess all, yet is not content with it, but by a holy distrust of jealousy will be set upon the heart which he possesses, and will seal it with himself, lest any of the love due to him escape, or anything get entry which might mingle with it. For he is not satisfied with the love with which the soul of his Sulamitess is filled, if it be not invariable, quite pure, quite solely his. And that he may not only enjoy the affections of our heart, but also the effects and operations of our hands, he will also be as a seal upon our right arm, that it may not be stretched out or employed save in the works of his service. And the reason of the divine lover's demand is, that as death is so strong that it separates the soul from all things, yea even from her own body, so sacred love which is come to the degree of zeal, divides and separates the soul from all other affections, and purifies her from all admixture; since it is not only strong as death, but it is withal bitter, inexorable, hard and pitiless in punishing the wrong done unto it, when rivals are entertained with it, as hell is hard in punishing the damned. And even as hell, full of horror, rage and cruelty, admits no mingling of love, so jealous love tolerates no mixture of another affection, willing that all be for the well-beloved. Nothing is so gentle as the dove, yet nothing so merciless as he towards his mate, when he has some feeling of jealousy. If ever you have taken notice, Theotimus, you will have seen that this mild bird, returning from his flight, and finding his mate amongst his companions, is not able to suppress in himself a certain sense of distrust, which makes him churlish and ill-humoured, so that at their first accosting he circles about her, murmuring, fretting, treading upon her, and beating her with his wings, although he knows well that she is faithful, and that he sees her in the pure white of innocence.
One day S. Catharine of Siena was in a rapture which did not deprive her of the use of her senses, and while God was showing her wondrous things, one of her brothers passed by, and with the noise he made disturbed her attention, so that she turned and looked at him for a single little moment. This little distraction, so unforeseen and sudden, was neither sin, nor disloyalty, but only a shadow of sin and resemblance of disloyalty: and yet the most holy Mother of the heavenly lover did so earnestly chide her and the glorious S. Paul so put her to confusion for it, that she thought she should have melted away in tears. And David, re-established in grace by a perfect love, how was he treated for the simply venial sin which he had committed in numbering his people?
But, Theotimus, he who desires to see this jealousy delicately and excellently described, must read the instructions which the seraphic S. Catharine of Genoa has made to declare the properties of pure love, amongst which she inculcates and strongly urges this; -- that perfect love, namely, love which has gone as far as zeal, cannot suffer any mediation, interposition, or mingling of any other thing, not even of God's gifts, yea, up to this extreme, that it permits not even the love of heaven, except with intention to love more perfectly therein the goodness of him who gives it. So that the lamps of this pure love have neither oil, wick, nor smoke, but are all fire and flame, which nothing in the world can extinguish. And those who carry these burning lamps in their hands, possess the most holy fear of chaste spouses, not the fear which belongs to adulterous women. Those have fear, and these also, but differently, says S. Augustine; the chaste spouse fears the absence of her husband, the adulterous, the presence of hers. The former fears his departure, the latter his stay: the one is so deeply amorous that she is extremely jealous; the other is not jealous, because she is not amorous: the one fears to be punished, and the other fears that she may not be loved enough; -- yet in sooth she does not precisely fear the not being loved enough, as other jealous persons do, who love themselves and want to be loved, but her fear is that she loves not him enough whom she sees so love-worthy that none can love him according to the greatness of the love which he deserves, as I have but just said. Wherefore she is not jealous with a jealousy of self-interest, but with a pure jealousy, which proceeds not from any cupidity, but from a noble and simple friendship; a jealousy which, with the love whence it proceeds, extends itself to our neighbour; for since we love our neighbour as ourselves for God's sake, we are also jealous of him, as of ourselves, for God's sake, so that we would even die that he may not perish.
Now as zeal is an inflamed ardour, or an ardent inflaming of love, it requires to be wisely and prudently practised; otherwise, under the cloak of it, one would transgress the limits of moderation or discretion, and it would be easy to pass from zeal into anger, and from a just affection to an unjust passion; wherefore, this not being the proper place to put down the conditions of zeal, my Theotimus, I tell you that for the practice of it you must always have recourse to him whom God has given you for your direction in the devout life.