As Dr. Benson points out, the term mysticism was not a common term until medieval times. Contemplation was used instead to describe contact or union with God. Christians who used contemplation include the anonymous author of The Cloud of Unknowing, the protestant reformer Martin Luther, St. Augustine, St. Teresa of Avila, St. Gregory Palamas and his “Sacred Quiteude”, and the sixteenth century monk Fray Francisco de Osuna.
The examples below are from St. Teresa of Avila and her writings in The Way to Perfection.
May the Lord teach this manner of prayer to those who do not know it for I confess, myself, that I never knew what it was to pray with satisfaction until he instructed me in this method.
… so the soul raises herself to a loftier region; she withdraws her senses from exterior objects … those who adopt this method almost always pray with their eyes shut … because it is making an effort not to think about earthly things.
[She] could never make any but vocal prayer, and, faithful to this, she had everything. Yet if she did not recite the words, her thoughts wandered so much that she could not bear it. But would that all had such mental prayer … I saw, that faithful to the Paternoster, she had reached pure contemplation.
In another example, Father Nicolas from the Eastern Orthodox tradition discusses how the hermit needs to detach himself from his body and mind as follows.
After years in a monastery or in a skete, detaching oneself from the world, the difficulty lies just in finding oneself alone, face to face with oneself, alone in control of your body and your mind. Because the mind is a wanderer, you know. Thoughts never stop following each other through your head, buzzing, preventing concentration, while in order to pray you need a great emptiness in your mind. After you’ve hunted out and punished all your vices, passions, faults — however trivial — you have to hunt out all your thoughts. You have to create an immense silence round you before you can reach the deepest silence in the depths of yourself. Continual prayer, repeating the same words of praise to the Lord; that’s what allows one to pray. It’s not a question of seeing God, but of being in God, and it’s not easy to contain in the narrow limits of your body the limitless spirit which is always trying to escape. That’s a hermit’s life, more or less.
In Russian monasticism, the method of Hesychasm was developed. It used the Prayer of the Heart or the Prayer of Jesus. Their approach is described in the quotes below.
Sit down alone and in silence. Lower your head, shut your eyes, breathe out gently, and imagine yourself looking into your own heart. As you breathe out, say “Lord Jesus Christ, have mercy on me.” Say it, moving your lips gently, or simply say it in your mind. Try to put all other thoughts aside. Be calm, be patient and repeat the process very frequently.
You know, brother, how we breathe, we breathe the air in and out. On this is based the life of the body and on this depends its warmth. So, sitting down in your cell, collect your mind, lead it into the path of the breath along which the air enters in, constrain it to enter the heart altogether with inhaled air, and keep it there. Keep it there, but do not leave it silent and idle, instead give it the following prayer: “Lord, Jesus Christ, Son of God, have mercy upon me.” Let this be its constant occupation, never to be abandoned. For this work, by keeping the mind free from dreaming, renders it unassailable to suggestions of the enemy and leads it to Divine desire and love….