The real presence of Christ under the aspects of the bread and wine consecrated at the Mass were believed by the Christians from the beginning. St. Paul comments on this--
cup of blessing that we bless,
is it not a participation in the blood of Christ?
The bread that we break,
is it not a participation in the body of Christ?"
(1 Cor. 10:16)
The martyrdom of St. Tarcisius illustrate the belief in the real presence of Christ in the Eucharist.
Christians were still persecuted in the 3rd century and what follows most likely happened during the persecution of Emperor Valerian. The Christians in order to escape notice used to worship in the catacombs which were and are underground cemeteries outside the walls of Rome. One day Tarcisius, a twelve year old acolyte, was present and since there was no deacon to bring communion to Christians in prison, he agreed to go.
On the way he met a group of pagan boys who invited him to play with them. He refused. Somehow he was identified as a Christian and when the boys saw he was carrying something "Holy mysteries" they demanded to see them. When he refused, they became a mob who turned upon him with fury. It is believed that a fellow Christian drove away the mob and brought Tarcisius to the Catacombs where he died. He was buried in the cemetery of St. Callistus and his relics are now in the Roman Church of San Silvestro in Capite. Pope St. Damasus in the 4th century wrote a poem commemorating Tarcisius martyrdom as follows:
a wicked group of fanatics flung themselves
on Tarcisius who was carrying the Eucharist,
wanting to profane the Sacrament, the boy preferred
to give up his life rather than yield up
the Body of Christ to those rabid dogs".