From the earliest days of the Church Christians have believed that Christ was present in the Eucharist, but they did so without theory or explanation. St. Paul simply admonished the Corinthians against sacrilegious reception (1 Cor. 11), but about 160 AD St. Justin wrote the first effort at an explanation, in his Apologia to Emperor Antoninus Pius.
For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh.
In the Middle Ages the character of this change was explored and systematically developed by theologians, especially St. Thomas Aquinas. Thus, the Council of Trent taught against the theories of some of the Reformers, that Christ is really, truly, and substantially present.
His Presence is real, because it has “real being.” This philosophical term conveys that it has actual existence, and not just mental existence (as a fictional creature, or a concept of a thing would).
His Presence is true. Truth refers to an accurate statement of a reality. The Blessed Sacrament is called Christ because it is Christ. It is not simply a symbol, as a flag is the symbol of a nation, or as a photograph is a representation of the individual shown.
Finally, His Presence is substantial. Even though our senses detect the appearances or properties of bread and wine, the substance is Christ, Who is wholly present, Body, Blood, Soul and Divinity, under each element, and any parts of them.