Teresa of Avila, answered from the texts and cited to the page.
I will tell you what I know of it, and I know it by being in it, not by reasoning. In the prayer of union the soul is asleep, fast asleep, as regards the world and itself. For the short time it lasts the soul is deprived of all feeling and cannot think on any subject even if it wished. No effort is needed to quiet the thoughts; the soul loves, and knows not how, nor whom, nor what it desires.
It has died entirely to this world, to live more truly than ever in God.1 I call it a delicious death. I cannot tell whether the body keeps enough life to go on breathing, and I believe that if it breathes, it does so unconsciously.1 The mind strains to understand what is happening, which is beyond its power, and is so astounded that it can hardly move, like one who falls into a dead faint with dismay.1
Do not mistake this for the drowsiness of the prayer before it. That one only seems to slumber; here the sleep is real as to the faculties.1 Nor is it anything we can bring about. I was struck by the Bride in the Canticles, who says the King brought her into the cellar of wine; she does not say she went of her own accord. The prayer of union is that cellar, where our Lord places us when and how He chooses, and we cannot enter it by any effort of our own.2
He asks of us only complete conformity of our wills to His and abandonment of all things. He does not need the faculties and senses to open the door, for they are all asleep; He enters the innermost depths of the soul without a door, as He came into the room where the disciples sat, saying Pax vobis.2 The soul comes out of it certain that God has been in it, and that certainty is the sign.
A soul without it has been united to God only by one of its powers, or has received one of the other favours He is accustomed to give.2 This concerns the Divinity, not any bodily presence of Christ. How we can be so sure of what we did not see, I do not know; it is His work, and I am certain it is so.2 There is a homely comparison I can offer, though the two things are very different.
It is like the preliminaries before a betrothal, when two people are allowed to see one another a little so as to decide. His Majesty, who knows whether the soul truly resolves to do His will in all things, gratifies His bride in return. He visits her and draws her into His presence that she may know Him better, and the visit lasts but a very little while.3
In that short time the soul sees in a secret way to what a Bridegroom she is betrothed, more than the senses and faculties could learn in a thousand years.3 But the soul is not strong yet. The devil will spare no pains to hinder the nuptials, so I beg you not to grow careless and to avoid every occasion of sin.3 The price is everything. God would have you keep back nothing from Him, little or great, and in proportion to what you have given will your reward be great or small. There is no more certain sign whether we have reached this prayer.4
In the prayer of union the soul is asleep, fast asleep, as regards the world and itself: in fact, during the short time this state lasts it is deprived of all feeling whatever, being unable to think on any subject, even if it wished.The Interior Castle, or The Mansions
I think the prayer of union is the 'cellar' in which our Lord places us, when and how He chooses, but we cannot enter it through any effort of our own.The Interior Castle, or The Mansions
We might compare the prayer of union to a visit, for it lasts but a very little while. There is no longer any question of deliberation, but the soul in a secret manner sees to what a Bridegroom it is betrothed.The Interior Castle, or The Mansions
God would have you keep back nothing from Him, little or great. He will have it all; in proportion to what you know you have given will your reward be great or small. There is no more certain sign whether or not we have reached the prayer of union.The Interior Castle, or The Mansions