|Left: a Ptolemaic Period (c.332-30BC) corn mummy (Ѻ), made of grain and mud, that was shaped and watered in the Khoiak festival month, representative of the re-born Osiris. Bread was made in these shapes and eating by the Egyptians, ritualistically, therein becoming reborn like Osiris. Right: the modern Catholic version of this, a child eating the Jesus wafer during Eucharist, and told that it is, in “reality”, the body of Jesus, the bread becoming Jesus via the mysterious process of transubstantiation.|
In 1623, Galileo published The Assayer (Il Saggiatore), which used so much atomic theory that it was called an Epicurean book; the church attacked the work per reason that it left no room for the doctrine of transubstantiation. 
Egyptian | Origin
The origin of Catholic practice of “transubstantiation” is an inherited version of the ancient Egyptian ritual of making the Osiris cakes (aka sacramental bread) (Ѻ), i.e. bread made being symbolic of the reborn or regrown god Osiris, which took place during the annual 30-day Khoiak festival. Each day of the Khoiak festival involved one of the steps of the process of making "corn mummies", a kind of old-fashion Chia Pet shaped like the god Osiris, so to say, that was watered and grew sprouts: 
Preserved examples of the Osiris cake moulds (Ѻ) date to 664BC; the British Museum describes the nature of these cakes as follows: 
“Osiris, supreme god of resurrection, was closely associated with the life-giving forces of nature, particularly the Nile and vegetation. Above all, he was connected with germinating grain. The emergence of a living, growing, plant from the apparently dormant seed hidden within the earth was regarded by the Egyptians as a metaphor for the rebirth of a human being from the lifeless husk of the corpse. The concept was translated into physical form by the fashioning of images of Osiris out of earth and grain. These "corn-mummies" were composed of sand or mud, mixed with grains of barley. As in this instance, the "mummy" is sometimes wrapped in linen bandages and may possess a finely detailed mask of wax, representing the face of Osiris.”
In 1911,Wallis Budge (1911) describes the mould cases for this divine bread as follows: 
“The moulds for the ‘divine bread’ were made of the wood of a red tree, and cakes were made from them in the forms of the [14 or 16] members of Osiris.”
In 1960, Martin Larson, in his “The Great Osiris” (Ѻ), summarized how the Osiris cakes was the original Egyptian Eucharist as follows:
“The mummies of the deceased would have inscriptions attached to them which read:
"I am Osiris. . . . Let not my limbs suffer corruption; let them not pass away; let them not decay; and let them be fashioned for me as if I were myself Osiris.... The mighty Osiris taketh possession of me. ... Behold, I am the god who is the Lord of the Underworld. . . . I am the Great One. . . . I have made myself whole and complete; I have renewed my youth; I am Osiris, the Lord of Eternity."
The rituals practiced by the Osirian mystery-cult were extraordinarily impressive. The great annual festival was celebrated in two phases, of which the first was public. It began at Abydos on the seventeenth Athyr, which is our thirteenth of November. This commemorated the death of Osiris, and its significance was emphasized by the fact that this was the very day on which the wheat was planted after the floodwaters of the Nile had receded. The god and the seed died at the same moment, for the grain was the god and the god was the grain. The resurrection of the god was the rebirth of the wheat. Man could be redeemed only through the divine sacrifice; the immortality of the communicant stemmed from that of his god. The sacrifice which was eaten and which was the eucharist was the dying god, reborn in the grain.
In this public phase of the Osirian passion play, the murder and dismemberment of the deity were depicted in moving scenes by highly skilled actors; to the audience it seemed that they were witnessing actual events, or a replica of them. This was followed by the search for, and the finding of, the body of Osiris by Isis, his resurrection from death, his triumphal return to power, his victory over the forces of evil, symbolized by Set, and his journey into the hereafter in his solar ship. This was known as the Coming Forth by Day.
Such was the first part of the Osirian festival. But within the temples there was an esoteric phase performed by the priests in which only the initiates could participate. All ancient writers were exceedingly secretive about this, for it was considered sacrilegious to reveal it. Nevertheless, from scattered fragments of information, we can reconstruct substantially what these secret rituals were.
Two days after the death of the god on seventeenth Athyr, the priests brought forth a sacred chest, which contained an image of Osiris; when this was revealed to the worshippers, they saw in it their resurrected god. In the temple at Dendera were found a series of inscriptions which describe how cakes of divine bread were made from the body of Osiris. These were the mysterious sacred cakes, which Plutarch calls the inward parts of the god and which were in fact the eucharistic bread. We see, therefore, that the publicly performed passion-drama depicted the earthly career of Osiris, but the secret ceremonials were eucharistic rituals, symbolizing the transmutation of Osiris into the grain and of the communicant into an Osiris.”
In 2003, Bartholomew Brewer, in his “The Mystery of Eucharist”, summarized how "Osiris cakes" became "Jesus wafers" as follows: 
“The doctrine of transubstantiation does not date back to the Last Supper as is supposed. Like many of the beliefs and rites of Romanism, transubstantiation was first practiced by pagan religions. In Egypt, priests would consecrate mest cakes which were supposed to become the flesh of Osiris [3100BC]. The idea of transubstantiation was also characteristic of the religion of Mithra [300BC] whose sacraments of cakes and Haoma drink closely parallel the Catholic Eucharistic rite. [In Roman religion], it was a controverted topic for many centuries before officially becoming an article of faith, which means that it is essential to salvation according to the Roman Catholic Church. The idea of a corporal presence was vaguely held by some, such as Ambrose, but it was not until 831AD that Paschasius Radbertus, a Benedictine monk, published a treatise openly advocating the doctrine of transubstantiation. Even then, for almost another four hundred years, theological war was waged over this teaching by bishops and people alike until at the Fourth Lateran Council in 1215AD, it was officially defined and canonized as a dogma.”
The ritual of eating of corn bread (Ѻ), resenting the body of Osiris (Ѻ), and the drinking of the beer (or wine), representative of Osiris conceptualized as the god who taught Egyptians to make barley and wine, in short, became re-written into the story of the last supper and the eating and drinking of the body and blood of Jesus Christ and the concept of transubstantiation. 
|A parody of transubstantiation, from the Awkward Moments Children’s Bible, by Horus Gilgamesh. |
In modern time, many children the world over are given the wafer of Eucharist and told simply that it is the transformed “body of Christ”, without being given the detail that this is actually symbolic of the “Osiris cakes” of bread made and eating during the annual Khoiak festival of ancient Egypt.
The following are related quotes:
“Since you have rejected our doctrine as containing things incredible, can you admit that of the Christians? Is not theirs still more contrary to common sense and justice? A god, immaterial and infinite, to become a man! to have a son as old as himself! This god-man to become bread, to be eaten and digested! Have we any thing equal to that? Have the Christians an exclusive right of setting up a blind faith? And will you grant them privileges of belief to our detriment?”— Constantin Volney (1791), The Ruins (§XXI: Problem of Religious Contradictions)
“I am what Richard Dawkins would classify as a level 6 ‘agnostic’ - agnostic in the sense that although I cannot prove that there is or isn't a god, there are an infinite number of possibilities I can't definitively disprove. I was brought up in a very devout Catholic family and ‘kicked against’ religion from the age of nine (when I began to ask questions about the ludicrous concept of transubstantiation in Catholicism).”— Philip Moriarty (2009), “Message (Ѻ) to Libb Thims”, YouTube, Sep 7
“Eating the wafer and turning into the body of Christ—transubstantiation; they tried to explain it to me, but I was like: ‘what do you mean that I’m eating him?’ ”— Martina (2015), polled response (Ѻ) on earliest memory (age 6-7) of being taught a religious concept that didn’t make sense, Jul
“By ‘eating the god’ in the form of the Eucharist, in a type of communion in leavened cakes, Osiris' divinity and immortality become that of the worshiper, and a spiritual communion through becoming one with their god-man. (cf. john 10:38). The sacred ritual of Osiris consisted primarily in the celebration of a Eucharistic rite, in which the advocate and followers of Osiris eat the flesh of their god in the form of wheat-cakes and drink of his blood in the form of a barley-ale (cf. Matthew 26:26, Mark 14:22, Luke 22:19, and I Corinthians 11:24).”— Christopher Reyes (2018), In His Name, Volume 5 (Ѻ)
1. (a) Budge, Wallis. (1911). Osiris and the Egyptian Resurrection, Volume One. P.L. Warner.
(b) Budge, Wallis. (1911). Osiris and the Egyptian Resurrection, Volume Two (Osiris cakes, 8+ pgs; divine bread, pg. 24). P.L. Warner.
2. Corn Mummy in Wooden Coffin (664-305BC) – HistoryPlace.com.
3. Brewer, Bartholomew F. (2003). “The Mystery of the Eucharist” (Ѻ), MTC.org.
4. Mojsov, Bojana. (2008). Osiris: Death and Afterlife of a God (corn, 4+ pgs.). Wiley.
5. Anon. (2013). Awkward Moments Children’s Bible (Ѻ) (last supper, pg. #). CreateSpace.
6. Hecht, Jennifer M. (2003). Doubt: A History: The Great Doubters and Their Legacy of Innovation from Socrates and Jesus to Thomas (pg. 319). HarperOne.
● Transubstantiation – Wikipedia.