Some points need clarification concerning The Lord's Supper and our present-day involvement with it.

In Matthew 26:28, Christ told his twelve disciples that the cup was "His blood poured out for many for the forgiveness of sins."

[You remember that another time Christ referred to a cup [of His upcoming torture and crucifixion?] which He initially considered either not partaking of nor drinking from in Matthew 26:39, with no allusion to pouring at that time].

Many presume that communion cups contain and/or should contain wine, but the Biblical record in the three gospels (Matthew, Mark, Luke) and I Corinthians chapter 11 does not specifically state the word "wine." Reference to a 'cup of wine' in the Bible is in Jeremiah 25:15 referring to God's wrath.

So, we see that "cup" has two meanings in the above passages:
ONE meaning is that the Lord does not directly refer to pouring out 'the fruit of the vine' out of that cup - whatever that was, and do not presume anything about that!) nor blood nor any other beverage within that "cup" (let's be SPECIFIC here!) but instead merely drink: (and here it specifically is): of the C U P . . . . . NOT mentioned drinking whatever contents there was therein.....and do not presume any euphemisms here about correspondingly sipping any liquids!
The OTHER meaning of cup was comprised of the [near-future] set of circumstances by which He would become The Perfect Atonement when hanging and dying on a cross with redemptive consequences for all penitent believers.

Cups are cups, fruit of the vine is fruit of the vine, and blood is blood - all are DIFFERENT from one another!

When Jesus fed the 4000 to 5000 with a few loaves of a kid's bread, He did not call that bonafide bakery or homemade bread His body, though He tried to convey to the general crowd the idea of He Himself being the euphemistic manna-like "bread" from heaven they should "eat." When He broke the unleavened passover bread at The Last Supper (recorded in ONLY a few verses of ONE chapter each in Matthew, Mark, Luke, and First Corinthians chapter 11 but not John), He did NOT state that "This BREAD is My body....." What He DID state was: "THIS is My body...." Similarly, He did NOT state: "This FRUIT OF THE VINE is My blood of the covenant" nor of course did He state: "This WINE is My blood of the covenant." It is as if the actual bread and actual cup accompanied His statements of body and blood euphemisms.....but themselves were not His body and blood.

So, was pouring out the cup (and ONLY the cup) rather than its contents some kind of PRE-atonement that actually redeemed the penitent both around the Last-Supper table and nowadays? Keep in mind that Christ had not in fact been crucified yet, and had not yet in reality been cursed by hanging on a cross nor died yet thereon thus THEN becoming our perfect sacrifice against all our sins.

Thus, it was neither then nor is it now that pouring out some cup and some cup atoned either for the sins of the twelve at the Last Supper - nor for us - who apparently repeated the tradition (as "oft" or whenever they would thereafter) which Christ merely gave to His twelve disciples sitting around the table back then (but which Saint Paul seems to equate with love feasts in his only-passing-mention FEW verses of First Corinthians chapter 11, which is only ONE chapter of ONE of his many epistles - thus plainly showing that he did not dwell on eucharist nor intend others to dwell on it to the extent of it becoming an overused-ritualistic highlight and culmination of each church service).

Not only are sins not forgiven by pouring out some cup back before the crucifixion nor pouring out some cup now afterwards), but some of the other things that Christ stated SEEM to indicate some kind of a pre-atonement.

For example, in John 15:3 Jesus told His disciple they they were "already made clean by the word" He spoke to them. "ALREADY made clean?" What does THAT mean? That utterance was declared before Jesus became the Perfect Atonement by dying on a cross!

"The blood" of "the prophets" was shed "from the foundation of the world" (Luke 11:50), and even though Revelation 13:8 records that the Lamb was slain "from the foundation of the world" (whatever THAT means), Revelation 17:8 says that saints names were also written "from the foundation of the world" (whatever THAT means) and Christ ONLY suffered at the end of the age - NOT repeatedly "from the foundation of the world" (Hebrews 9:26).

In Mark 2:5 and Luke 5:20, Jesus told the paralytic that "your sins are forgiven." Really? BEFORE Christ became The Sacrificial Atonement by dying on a cross? On what basis was atonement forgiveness bestowed by the pre-crucified pre-atoning Jesus....not to mention a paralytic who had not sipped the Last-Supper cup nor drunk Christ's blood? The Sacrificial Atonement had not even atoned yet to the point of death when He declared entrance into purgatory (excuse me: Paradise) for the dying thief on a nearby cross, which penitent thief also neither ate Christ's fleshy blody nor drunk Christ's blood nor sipped any Last-Supper communion cup.

Since Christ [the timeless and absolutely-perfect son-of-man] forgave sins before He was crucified dead - (at and only at which time He became the Atoning Sacrifice) - WHY did He die on a cross for the purpose of forgiving sins? If then since Jesus could (and did) simply verbally declare forgiveness of sins to someone, how come He had to die on a cross to forgive sins of people? Why the additional (and needless?) crucifixion for that purpose?

While we're on the subject, exactly what forgave the Old-Testament patriarchs their sins: the Jewish/Hebrew animal sacrifices of sheep, goats, and lambs done at that time for them....or instead/along-with Christ's future death on a cross?

Clearly, once the crucifixion-to-death Act was done, those from THEN on were officially (and henceforth!) forgiven by that Act. But before It happened?

I never did like the implications in the movie Minority Report where people were arrested WAY before they even thought of doing their criminal acts. As they say, "It ain't over till the fat lady sings." It hasn't happened until it actually happens. Executing people WAY before they ever even think of committing gross illegalities would be more than enough basis to abort babies before they ever get a chance to commit serious felonies. But of course, such mis-thinking is totally absurd and insane.

Pre-atonement is like getting paid for a job the POTENTIAL employee has not done yet. It would be ludicrously risky for any employer to take such a chance of paying people BEFORE they do they work with no guarantee that, once paid, those people would actually do the work to deserve the payment.

Clearly, even after reading Jewish sacrificial-atonement-explanations/precedents doctrine in the New-Testament book of Hebrews (proving Jesus was The Ultimate Animal Sacrifice - the Prophesied Passover Lamb), there is mystery in Christ's pre-atoning statements of sins forgiven. However, the Perfect God and Creator has been known to "do it His way" in everything. HE calls the shots however HE chooses. And with the infinite power and perfection He alone has, He certainly has "the right," and WHO can stop or question Him as to WHATEVER He says and does? Perhaps He felt that He had to confine and inflict Himself with a crucifixion so embarassing, so humiliating, so degrading, so painful, so obnoxious, and so despicably ugly....to make up for allowing The Tree of The Knowledge of Good and Evil into the Garden of Eden, then giving both Eve and Adam access to it, with the Murphy's-Law likelihood that they would access it - followed by all the resulting misery, poverty, disease, suffering, difficult-to-fully-describe horrible atrocities characteristic of humans and their unspeakable inhumanity-to-man misbehaviors with disasterous consequences from that time on.

In summary, it is neither ANY poured-out communion cup (containing whatever) that forgives our sins and thus saves us, nor (THINK carefully now without prejudicial knee-jerk reaction!) even our believing that Christ's one-time once-for-all atoning sacrifice of Himself completely erases all our sins and all the penalty for those transgressions. What redeems us is neither the justifying mechanism of faith given to us as a gift from God, nor the meritorious justifying response of good works we are enabled to do as a gift from God (being that every good and perfect gift - namely, faith and works - comes down from The Father of Lights, according to James).

What saves us instead is:

NOT any [blasphemously-uttered] hideous claim that we "sacrifice Christ's body back to Him" (how arrogantly impudent!) with the [ridiculous] presumption of atoning or co-atoning for our own sins ourselves.