According to St. Augustine, the first heresy amongst the disciples of Christ was the failure to believe that Christ truly gives us His precious Body and Blood in the Eucharist:
“For certain things in the scriptures were seeming hard, while they were obscure; when explained, they have been softened. For even the first heresy in the disciples of Christ, as it were from the hardness of His discourse. For when He said, ‘Except a man shall have eaten My flesh and shall have drunk My blood, he shall not have life in himself:’ they, not understanding, said to one another, ‘Hard is this discourse, who can hear it?’ Saying that, ‘Hard is this discourse,’ they separated from Him: He remained with the others, the twelve. When they had intimated to Him, that by His discourse they had been scandalized, ‘Will ye also,’ He saith, ‘choose to go?’ Then Peter: ‘Thou hast the Word of life eternal: to whom shall we go?’ Attend, we beseech you, and ye little ones learn godliness. Did Peter by any means at that time understand the secret of that discourse of the Lord? Not yet he understood: but that good were the words which he understood not, godly he believed. Therefore if hard is a discourse, and not yet is understood, be it hard to an ungodly man, but to thee be it by godliness softened: for whenever it is solved, it both will become for thee oil, and even unto the bones will penetrate.”
-Saint Augustine, On the Psalms 50.22
We must follow the example of Peter and believe the words of Christ, hard though they be. If Christ says that it is His Body and Blood in the Eucharist, then it is so. As God's speech has the power to bring things about in creation, so does He have the power to change the very bread and wine He holds in His hands at the Last Supper into His Body and Blood. When this truth is realized, it will, according to Augustine, penetrate even to your bones. Your entire life will be changed. Believe Christ and see the depth and beauty and wonder of the Eucharist!