With a few minor adaptations, here is what I wrote in the 1990s:
My call is to be in the world but not of the world. This is not a new or different idea; rather, it is an emphasizing, a kind of "underlining," of every Christian's call. The uniqueness of this emphasis is in its monastic imagery. I find it helpful to recognize that within me is a "place" set apart for and consecrated to God. This place of consecration is sacred and inviolate, for the God of all dwells therein.
The word "cloister" speaks of total consecration. Those who enter a traditional physical cloister make a tangible break from the world. Compromise does not fit well in a cloister, nor does lukewarmness, nor does complacency. The cloistered life is meant to be absolute. A nun living in a cloister has made a decision to live for God. She has made a break.
A Christian living in the world is also called to make a decision to live for God, but the break is not so clean. The world is persistent in its tugs on the heart trying to live for God. Therefore, we need support in our struggles to surrender our lives to God and to resist the world's allurements. This is where the imagery of the cloistered heart can be of help.
"It is best not to consider whether or not one is called to the cloister; that is not the point. If the cloister is in a man's heart, it is immaterial whether the building is actually there. The cloister in a man's heart means only this: God and the soul." (from Warriors of God by Walter Nigg, NY, Alfred A. Knopf, 1959, p. 13)
A cloistered heart may be married or single, nurse or engineer or homemaker, yet the heart can be cloistered. My cloister is not made of bricks and stones, but of God's holy will in which I have chosen to live. The will of God forms for me a "cloister grille," through which I may view and respond to all people, all circumstances, all things that make up the world in which I live. My commitment to God does not conflict with family life, but rather enhances and empowers it.
Many years ago, another had this same kind of vision. St. Jane de Chantal, when she was yet a laywoman (widowed with four chldren), imaged her spiritual world with monastic imagery, and took the Virgin Mary as the Abbess of the cloister of her own heart.
I ask for her intercession and for that of St. Francis de Sales, who encouraged Jane in her monastic imagery. May they pray for all of us who wish to live in the world as "cloistered hearts."
(painting of Kloster by Eduard von Kallee, US public domain)