Generally, Christian mysticism has been described as a form of spirituality that sets as its goal unity with God. Mystics often have reached a level of contemplation at which they cannot describe their experiences but instead use colorful language and poetry to convey the gist of it to those who have not experienced their ecstasy. They tend to see the whole world as charged with divine glory, and they sometimes actively change the world by their vision of justice and love.
Union with God can mean complete absorption, ultimately losing one’s identity in God. Communion, on the other hand, suggests a kind of loving relationship of two persons who remain distinct in spite of a unity of purpose, feeling, or knowledge. But not all mystics use either of these terms.
— Bradley P. Holt, Thirsty for God: A Brief
History of Christian Spirituality