That this may be understood, it is necessary to explain the difference between a union of the powers and essential union. Either of them may be transitory, and for a few moments only, or permanent and lasting.
The union of the powers is that by which God unites the soul to Himself, but very superficially; it is more properly a contact than a union.
It is nevertheless united to the personal Trinity according to the different effects peculiar to each member of it; but always as if to distinct persons, and by an intermediate operation. This operation serves both as a means and an end, the soul resting in the union thus experienced, without supposing that there is anything beyond.
This union is accomplished in order, in all the powers of the soul, and is sometimes perceived in one or two of them according to the designs of God, and at others in all three together. This constitutes the application of the soul to the Holy Trinity as to distinct persons.
When the union is in the understanding alone, it is a union of pure intellect, and is attributed to the Word as a distinct person.
When the union is in the memory, which is effected by an absorption of the soul into God, and a profound forgetfulness of the creature, it is attributed to the Father, as a distinct person.
And when it takes place in the will alone, by a loving joy without sight or knowledge of anything distinct, it is a union of love, and is attributed to the Holy Spirit, as a distinct person. And this latter is the most perfect of all, because it approaches nearer than any other to essential union, and is generally the road by which the soul arrives at it.
All these unions are divine embraces -- but they are not the Kiss of His mouth.
These unions are of two sorts, the one transitory and very short lived, the other permanent and sustained by the perpetual presence of God, and a sweet and tranquil love, which continues in the midst of everything.
Such, in a few words, is the union of the powers, which is a union of betrothal; it implies the affection of the heart, caresses and mutual presents, as is the case with the betrothed, but not the full enjoyment of its object.
Essential union and the kiss of His mouth is the spiritual marriage, where there is a union of essence with essence, and a communication of substance -- where God takes the soul for a spouse and unites Himself to it, no longer by persons nor by any act or means, but immediately, reducing all into unity and possessing it in His own unity. 
Then it is the kiss of His mouth, and real and perfect possession. It is an enjoyment which is neither barren nor unfruitful, since it extends to nothing less than the communication of the Word of God to the soul.
We must remember that God is all mouth, as He is all word, and that the application of this divine mouth to the soul is the perfect enjoyment and consummation of the marriage by which the communication of God Himself, and of His word, is made to the soul.
This is what may be called the apostolic state, in which the soul is not only espoused but fruitful, for God, as mouth, is some time united to the soul before rendering it fruitful of His own fecundity.
There are some who maintain that this union cannot take place until the next life, but I am confident that it may be attained in this, with this reservation, that here we possess without seeing, there we shall behold what we possess.
Now I say, that while the view of God is in addition to our glory, without which it would be incomplete, it does not, nevertheless, constitute essential beatitude; for we are happy from the moment we receive the supreme Good, and can receive and enjoy it without seeing it. We enjoy it here in the night of faith, where we have the pleasure of enjoyment without the satisfaction of sight; there, we shall have the clear vision of God in addition to the happiness of possessing Him. But this blindness hinders neither the true possession nor the veritable enjoyment of the object, nor the consummation of the divine marriage, any more than it does the real communication of the Word to the soul.
This is far from imaginary, as will be attested by every person of experience. 
The present is a proper opportunity to resolve the difficulty of some spiritual persons who think that when the soul is united with God in an essential union, it can no longer speak of Jesus Christ and his interior states, the soul having passed through and left that state. I agree with them entirely, that union to Jesus Christ has preceded for a long time the essential union, since union with Him as a person, took place during the union of the powers; and further, that the union with the God-man Christ Jesus is the first of all, and occurs at the very beginning of the illuminated life. But as regards the communication of the Word to the soul, I say that the soul must first have arrived in God alone,  and been there established in essential union, and by the spiritual marriage, before the divine communication can be made to it; as the fruits and products of marriage can only appear after its consummation.
All this is more real than can be expressed; and in the fact that God here possesses the soul without interruption, we may trace the difference between essential union and every other kind. When united with the creature, we can only enjoy it by intervals, because the creature is without; but the enjoyment of God is permanent and lasting, because it is within, and God being our final end, the soul can incessantly pour itself into Him as into its goal and centre, to be there mingled and transformed without ever again coming out.  Just as a river, which is composed of water derived from the sea, and quite distinct from it, finding itself away from its original, endeavors in various ways to reach the ocean; which, having done, it loses and mixes itself with it, just as it was before it left there, and can no longer be distinguished from it.
It is further to be observed, that God, in creating us, made us participants of His being and fit to be reunited to Him; at the same time bestowing upon us a tendency towards such a reunion.  He has imparted a similar trait to the human body in respect to man in a state of innocence, drawing it from man himself, that He might give it this inclination to union, as to its origin. But as this takes place between gross, material substances, the union can only be material and very restricted, because it occurs between solid and impenetrable bodies. This may be illustrated by the attempt to unite two metals of very different qualities by fusing them together; they never can be perfectly united on account of their dissimilitude; but the nearer alike the two metals are, the more readily they mix. On the other hand, mix two glasses of water, and the two immediately become so mingled as to be undistinguishable. Thus, the soul, being perfectly spiritual in its character, is altogether fitted to be united, mingled and transformed in its God.
This may be illustrated by the union of salt and water: when a lump of rock salt is thrown into water, there is union between the two, because they are on all sides united; but when the salt is liquefied, dissolved and vanished, then there is union and admixture.
There may be a union without any intermixture; such is the union of the powers. But the intermingling is the essential union; and this union is absolute, being of all in the all.
It is only to God that the soul can be thus united, because such is its nature by creation. This is what Saint Paul calls being changed into the same image (2 Cor. iii.18), and the Savior, oneness (John. xvii.11, 21).
Now this takes place when the soul loses its proper subsistence to exist only in God; by which is meant mystically, the loss of all self-appropriation, and a loving and perfect sinking of the soul into Him, and not that essential despoiling of its intimate existence implied in the hypostatic union. It is as when a drop of water is let fall into a cup of wine; it loses its own appropriate form and character, and is apparently changed into wine; but its being and substance always remains entirely distinct; so that, if it were the will of God, an Angel could, at any time, separate the identical drop. In the same way, the soul may always be separated from God, though with great difficulty.
This, then, is the lofty and intimate union that the Spouse so pressingly demands at the hand of the Bridegroom. She asks it of Him as though she was addressing another; an impetuous sally of love, giving vent to her passion without particular thought as to whom she was speaking. Let Him kiss me, says she, since He can do it, but let it be with the kisses of His mouth; no other union can content me; that alone can satisfy all my desires, and that is what I demand.