When the soul realizes that all its goodness is, in fact, not its own at all, it is ready to enter more profoundly into itself. As you leaves the third mansions for the fourth, you begin to realize a new horizon has been crossed. From here until complete union with God, the soul lives in the "spiritual mansions."
This does not mean that we withdraw completely into the "mind" or the "spirit," but rather that the soul has come to draw all its joy, peace, and strength from the Source itself. Teresa speaks of the first three mansions as states of great effort on the part of the soul--it gains equilibrium, peace, and enjoyment from its good works and prayers. This is true peace and joy; however, the consolations of the first stages of the spiritual life are dependent on many "secondary causes." The consolations of the fourth mansion emanate directly from God himself.
It is clear why humility is the only door into the interior mansions: We could never "attain" or "earn" the graces found here. God moves when and where he will.
But what is this humility Teresa urges us to cultivate? What is the key to the fourth mansion?
"The first proof that you possess humility is that you neither think you now deserve these graces and consolations from God, nor that you ever will as long as you live."
This does not mean we give up on the efforts made in the beginning--regular prayers, service to the Church, fidelity to our vocation--but rather that we give up on our own pride of life. Instead of desiring the bliss of the spiritual mansions, we in fact would rather suffer for our sins. (This reminds me of St. Gianna Molla who, after the doctors thought she had slipped into her final coma, awoke to say she had been sent back to suffer for sins. Even after seeing the gates of heaven, she had desired to suffer more before being given eternal joy.)
Another sign of humility is the renunciation of the world. God sends his greatest graces to those in whom nothing blocks his action. If our souls obsess themselves with schedules, costs, shopping, grades, children's grades, etc... there is very little room for the Lord to speak. But if, after performing as best we can the work he demands of us, we leave the rest to his mercy, he can manifest that mercy more abundantly. This is renunciation of the world.
Finally, to reach these interior mansions, we have to lay our plans for reaching them at the feet of Christ. Love is the only way to intimacy with God:
"I only wish to warn you that to make rapid progress and to reach the mansions we wish to enter, it is not so essential to think much as to love much: therefore you must practice whatever most excites you to this. Perhaps we do not know what love is, nor does this greatly surprise me. Love does not consist in great sweetness of devotion, but in a fervent determination to strive to please God in all things, in avoiding, as far as possible, all that would offend Him, and in praying for the increase of the glory and honor of His Son and for the growth of the Catholic Church. These are the signs of love..."Notice that love has more to do with submission to God's work on earth (fleeing sin, prayer for souls and the Church) than with our feelings about the matter. The soul that loves may from time to time feel delight in love--that is the gift of God and a foretaste of heaven--but it does not love in order to feel that delight.
The gift of the fourth mansions is the prayer of recollection. From its center, where God dwells, the soul hears directly "the voice of the Shepherd." Teresa compares that voice to the music of a flute or the voice of a loved one. The soul is overwhelmed with joy and may weep--as if it had just seen someone it had thought dead. It is a direct encounter with the living God.
"The after effects on the soul, and the subsequent behavior of the person, show whether this prayer was genuine or no: this is the best crucible by which to test it." The soul will grow in love--of the Father, Christ, the Church, and the world--and its actions will radiate a strength inexplicable by its natural powers or talents.