St. Teresa of Avila spent much time writing and mediating on the spiritual life, striving in her own life to be as close to Jesus as she possibly could.
She wrote extensively about it in her book, Interior Castle, which talks about the seven "mansions" of the spiritual life.
The seventh mansion features what she calls a "mystical marriage."
The Catholic Encyclopedia summarizes it by explaining, "the term mystical marriage is employed by St. Teresa and St. John of the Cross to designate that mystical union with God which is the most exalted condition attainable by the soul in this life."
Mystical marriage
St. Teresa of Avila uses much more poetic words to describe this intimate union:
[S]piritual marriage with our Lord, where the soul always remains in its center with its God. Union may be symbolized by two wax candles, the tips of which touch each other so closely that there is but one light; or again, the wick, the wax, and the light become one, but the one candle can again be separated from the other and the two candles remain distinct; or the wick may be withdrawn from the wax. But spiritual marriage is like rain falling from heaven into a river or stream, becoming one and the same liquid, so that the river and rain water cannot be divided; or it resembles a streamlet flowing into the ocean, which cannot afterwards be disunited from it.
She then explains how this intimate union of a soul can also benefit others in the Church:
For from the bosom of the Divinity, where God seems ever to hold this soul fast clasped, issue streams of milk, which solace the servants of the castle. I think He wishes them to share, in some way, the riches the soul enjoys; therefore from the flowing river in which the little streamlet is swallowed up, some drops of water flow every now and then to sustain the bodily powers, the servants of the bride and Bridegroom.
All of us are called to union with Jesus Christ and in Heaven we will fully achieve that union for all eternity.
At the same time, we can also strive to anticipate that union while still on earth.
This type of mystical marriage is not something easy and it often takes a lifetime before someone can be united to God in this way.
While most of us likely will never experience such a union, we are invited to go deeper into the Interior Castle and die to the world, so that we may live more fully in Jesus Christ.