Many denominations, both orthodox and heretical, believe that the bread and wine consumed during communion are, to some extent, literally Jesus' flesh and blood. One of the more common arguments to "prove" this from the Bible is that when Jesus broke the bread and shared the wine, He declared "this is my body" and "this is my blood", allegedly solidifying the literal nature of communion.
A major problem with this is that Jesus used many strong statements that are clearly not intended to be literal. "...unless one is born again, he cannot see the kingdom of God." (John 3:3, emphasis mine). "...I am the door of the sheep." (John 10:7, emphasis mine). "I am the true vine, and My Father is the vinedresser." (John 15:1). These are all statements Jesus made that are clearly not literal. We do not literally need to be born again. Jesus is not literally a door through which we, as literal sheep go. Jesus is not literally a vine. Most people do not interpret these things as literal statements, except, ironically, those who, having heard this argument, will stubbornly make insane excuses for why they can be considered literal.
There is another metaphor the Bible uses concerning blood: To be washed in Jesus' blood (Revelation 1:5) or for those who get saved during the Tribulation to wash their robes white in His blood (Revelation 7:14). No one believes we must literally be washed in Jesus' blood (I hope...). This is clearly a metaphor representing the fact that by the shedding of Christ's blood on the cross, God forgives our sins. The same is true, and I would say axiomatically so, for the bread and wine being Christ's body and blood. There is nothing in the text that compels us to believe Jesus was speaking literally (and plenty that strongly suggests the opposite), there are early references to it not being literal, there's just no sensible reason to believe this is the one time Jesus spoke literally, in spite of the Bible actually telling us even His disciples were surprised on the rare occasions He did so (John 16:29).
Now, for some religions, this really isn't a huge issue. It's a little weird, and certainly not justified by the Bible, but on the whole it's just a misunderstanding believers will have to account for later. But for some religions, this is literally a Gospel issue. The Council of Trent, for example, declared "If any one denieth, that, in the sacrament of the most holy Eucharist, are contained truly, really, and substantially, the body and blood together with the soul and divinity of our Lord Jesus Christ, and consequently the whole Christ; but saith that He is only therein as in a sign, or in figure, or virtue; let him be anathema." In other words, you are not a Christian, and guaranteed to go to Hell, if you do pretty much what I have done with this post. That's not just heresy, that is an alteration to the Gospel, which is so dangerous, Paul said that not even the Apostles or an angel had the authority to do it, and that if anyone does do it, they are anathema (Galatians 1:8).
I wish to be clear, to all Christians who nevertheless believe this false doctrine, while it is factually wrong, it is not an issue worth dividing over. But those who make this a Gospel issue need to repent, and come out of the Churches that do so. Salvation does not depend on continuously faking cannibalism, it depends on faith in Jesus Christ. One need no more literally consume Christ than be literally washed in His blood.
Recommended song: Alan Jackson - Are You Washed in the Blood of the Lamb?