"The mystical movement, the classical form of Roman Catholic piety, developed as a reaction against formal and mechanical sacerdotal ritual and the dry Scholasticism in the church of the day." (Cairns, 250)
At this point, church was no longer for everyone, but it was acted out by clergy and spectated by lay people. They got the mass, but they got no Word of God outside of Latin and no Word of God during the week. People were starving for interaction with Christ. Mystical movements resulted from this famine.
In 1348 and 1349, the bubonic plague caused the Black Death that killed many people in illness. Peasants also revolted in 1381 following the teachings of John Wycliffe. Also, the two or three popes ruling made consistency impossible for the holy lands. These people needed spiritual leadership and interaction with God.
There were Latin mystics and Teutonic mystics. Latins had a more emotional outlook on life. They emphasized emotional experiences of Christ. These were the charismatics. I would not necessarily agree with them as I believe signs and wonders have stopped, but I would still accept them as brethren. Bernard of Clairvaux was a Latin mystic, Augustinian monk, and influence on the reformers. Catherine of Siena was also of this ilk. She was the one that urged Gregory 11 to move back to Rome.
The Teutonic mystics took a more philosophical approach to life. These were the emergents who might have had cool ideas but flirted with heresy. Meister Eckhart was of this group and envisioned a kind of worship where people tried to reach unity with the Godhead. This is gnosticism, and it is never new. It leads to pantheism, and the Church condemned Echkart's teachings after he died.
Condemned or not, a group of Dominicans formed the Friends of God to carry on Eckhart's tradition. They wrote Theologia Germanica, and emphasized the inward experience of God more than the outward motions of worship. Unlike Eckhart, this was truly Christian. It struggled with monophysitism in not just Christ but as the goal of believers; they were to fuse their souls with the divine. Just the same, the Friends of God and their teachings soothed Martin Luther as he struggled with his salvation. This movement also produced Thomas a Kempis, another man that reformed people love. He is credited with writing Imitation of Christ. He gave a more practical approach to the Friends of God. I might have to read some of this theology myself to see what is not right and what true Christians can believe.
Mystics can be solid Christians, but their emphasis on the inner soul can be dangerous, especially if a person looks inward and does not have the Holy Spirit, aka, is not saved. It was a good alternative to the play acting in the churches where people were not learning or experiencing Jesus properly. They read the Scriptures more and reached out to communities. The ideal would be for them to gather as a church on Sunday, read the Bible in vernacular language, and help people understand and interact.