Rudolf Steiner, September 5, 1924 [from The Book of Revelation and the Work of the Priest, lecture 1]:
Thus in those ancient Mysteries, in times when the cosmic language, not the human language, was current among human beings, people sought to make contact with the gods who then descended into the Mysteries and who on each occasion made holy once more the act of consecration of man. On each occasion, too, an understanding of apocalyptic things was bestowed upon the human beings who celebrated the act of consecration of man. This is how the great truths were taught in those ancient times when being in the midst of the act of consecration of man meant being filled with apocalyptic substance. The act of consecration of man is the path of knowledge; apocalypse, revelation, is the content of the holy knowledge.
Now we come to the semi-ancient Mysteries, those Mysteries of which at least a faint shimmer remained in historical times. (Of those that I have described as the ancient Mysteries nothing historical remains; they can be researched only by spiritual science.) The time had come when the gods withdrew from human beings and no longer descended into the Mysteries with their own being; but they continued to send down their forces. It was the time when the act of consecration of man was to receive through the Transubstantiation that shining light of the divine that was intended always to illumine the act of consecration of man.
The Transubstantiation was now no longer accomplished through learning from astrological observation of cosmic processes about which substances and forces should flow into the celebration of the Transubstantiation. Now the secret was sought in a different way. The inner nature especially of what the old alchemists called 'leavens' was taken up. A leavening agent is a substance that has reached a specific age while having gone through various stages of bringing about transformations in other substances without changing its own substance. To make an everyday comparison we need only think of how bread is baked. The principle is the same. You save a small piece of your previous batch of dough and add it to the new batch as a leavening agent. Imagine how in the semi-ancient Mysteries age-old substances, having retained their own inner substantiality through the ages while other substances were undergoing transformation, were stored in holy vessels that were themselves ancient and holy objects, venerable objects in the Mysteries.
From these ancient vessels were taken the substances that were the leavening agents with which the Transubstantiation of the old, still holy alchemy was performed. People in those days knew: The initiated priest understands the transformation, the Transubstantiation taking place through the forces preserved in the substances; he knows that they send forth sun-radiance in the holy quartz-crystal vessels. What was looked for in them, the reason they were needed, was that they were seen to be the celebrant's organ of perception for apocalypse, for revelation.
The following was something that took place in those semi-ancient Mysteries: The priest was tested at the moment when he approached the holy place and the ancient leavening agents began to transform the substances in the holy quartz-crystal vessels in such a way that he could see in those vessels how the substances sent forth sun-radiance. The vessel containing the little sun was a monstrance. It was a Host such as can only be recreated in imitation nowadays. The moment when he saw the sun-radiance of the Host was the moment in which he became a priest in his inner being. (Plate 1)
Nowadays in the Catholic church, everyone going to church sees the consecrated Host, for now it is no more than a symbol for what it once was. In those days, however, only those individuals were genuine priests who beheld the consecrated Host in that they saw a sun-radiance in the substances that had been preserved. At such a moment their knowing was open to receive that which is apocalyptic.