It is NOT a true sacrifice because Jesus didn't come to this earth so that we humans would have something to offer to God as a sacrifice for our sins in the same manner that the Jews brought their animals to the sacrifices at the Temple.
It's also not a true sacrifice because the catechism states that it's bloodless, which invalidates it as a sacrifice. Without the shedding of blood, there is NO forgiveness of sins. If there is no blood in the sacrifice of the mass, as the CCC states there isn't, then it's just participation in a useless killing. Also, they are now forced into a position of telling us just where the blood of Christ that they claim they are eating, comes from.
Also Jesus died on a cross, He was NOT offered up by a human priest on an altar, so that even the method of allegedly participating in the sacrifice of Jesus is wrong.
Jesus came to pay the penalty for our sin Himself, of His own accord. He stated clearly that no man takes His life from him but that HE lays it down or His own free will.
The Passover was a commemorative meal of remembrance of what God did in delivering the Israelites out of the hands of the Egyptians, and likewise, the Last Supper is, too.
I was simply instituted BEFORE the event instead of during or after.
Isaiah 1:11
I have had enough of the burnt offerings of rams and the fat of fed beasts [without obedience]; and I do not delight in the blood of bulls or of lambs or of he-goats
...gimme dem wafers and wine instead for a SACRIFICE!
Oh...
...ignore the following...
Hebrews 10:10
And by that will, we have been made holy through the sacrifice of the body of Jesus Christ once for all.