"Mystery" in this proper sense, is derived from muein, "closing the eyes" or "closing the mouth". In gaining ordinary knowledge it is necessary to open one's eyes in order to grasp the object and to open one's mouth in order to communicate with other persons and have one's insights tested. A genuine mystery, however, is experienced in an attitude which contradicts the attitude of ordinary cognition. ... Mystery characterizes a dimension which "precedes" the subject-object relationship. The same dimension is indicated in the "closing of the mouth". It is impossible to express the experience of mystery in ordinary language, because this language has grown out of, and is bound to, the subject-object scheme. I mystery is expressed in ordinary language, it necessarily is misunderstood, reduced to another dimension, desecrated. This is the reason why betrayal of the content of the mystery cults was a blasphemy which had to be expiated by death.The main reason I love Tillich's theology so much is because it is so intensely spiritual. While many more traditional theologians shy away from talk about the divine in our life, Tillich says extremely bold things about things like miracles, prophecy, ecstasy and, as here, mystery. Again, his isn't a theology of cool reflection, it is all the time a theology in the face of despair, a theology that struggles to make life meaningful.
Systematic Theology I, 108-109.
The etymological background to the word mystery is well-known, but it is seldom taken to this breathtaking conclusion. Tillich is secretly very polemical, only he very seldom bothers to name those he argues against. Here I think he is at the same time addressing fellow theologians, as well as preachers in the churches. He is attacking the tendency to say things about that which cannot be said. And in what way! "If you would have been members of an ancient mystery cult and talked about the divine the way you do, they would have killed you!"
I think this may be the reason that I, as student of the church fathers, feel so at home in Tillich's theology. They too knew where the line should be drawn for what could be said, and they never tried to go beyond that. And we all know what happens when humans try to formulate that which cannot be formulated: this is when churches split, people are anathematized and religious wars are fought.
Tillich's warning is still very much needed.