What is the Lord’s Supper and why do we believe what we believe which is that we are eating Christ’s body and blood in, with and under the bread and wine? Because that is what Jesus says we are doing. On the night in which He was betrayed, on Maundy Thursday, Jesus celebrated this Passover with His disciples.
So, as Jesus was celebrating the Passover meal with His disciples, at one point in the meal. He took bread, blessed it and gave it to His disciples. With the bread in His hand He told them to take and eat the bread which was His body (Matthew 26:26). Because Jesus was the Lamb of God who takes away the sin of the world, it was important that as He offered Himself as the sacrifice for the sins of the world, that those who participate in the benefit of His sacrifice also eat of the sacrifice; thus, Jesus declares that the bread being eaten actually is His body. Jesus does not say the bread represents nor is changed into, but is actually His body.
After He distributed His body/bread to His disciples, He then took the third of four cups of the Passover, the cup of Redemption, blessed it and passed it around for His disciples to drink with the words, “Drink of it, all of you, for this is my blood of the covenant, which is poured out for many for the forgiveness of sins” (Matthew 26:27b-28a). Notice here again, Jesus did not say the cup of wine symbolized or changed into, but was His blood. And here again, as they drank His blood/wine, it became a part of them. Thus, as those who eat Jesus’ body and drink Jesus’ blood, the body and blood of the Lamb of God who takes away the sin of the world, they are participating in Jesus so He becomes a part of them. As He is a part of those eating and drinking, His life becomes theirs, His suffering and death become theirs, and His resurrection and eternal life become theirs.
Notice also, that Jesus never explains how this is happening, our eating and drinking His body and blood (as we explain) in, with and under the bread and wine. He simply tells us this is what we are doing; thus, we eat and drink in faith believing the Word of Jesus whose Word does what it says and gives the gifts He says it gives. And we rejoice and say, “Amen,” God is faithful. “Amen,” gift given, our being given to, thanks be to God and to Him be the glory.