Some of the apostles, particularly John and Mathew were involved in writing their Gospels, if not directly, at least indirectly. John 6 teaches “This IS my body” "This IS my blood.
St. Paul teaches this in one of his epistles. The very early writers, called The Church Fathers taught this. As centuries passed, it has been taught in every age. In about the 11th Century St. Thomas, using the meta-physics of Aristotle explained the Mystery that had been taught for at least a millenium and called the change of bread and wine into the body and blood of Jesus Christ transsubstantiation; a new word trying to explain, what had been held as fact for centuries and which has continued to be held by the Catholic and Orthodox Churches to this very day.
A lot of our modern doubt occurs because the notion of substance in modern science is quite different from the notion of substance in the meta-physics of St. Thomas. I am no great shakes as a meta-physician so perhaps someone else can go from here.