Protestant
Isn't the Mass a re-sacrifice of Christ?
The Mass does not repeat or add to Calvary; it makes present, in an unbloody sacramental manner, the one sacrifice Christ offered once for all, so that the same single offering reaches every generation.
Catholic teaching
Hebrews teaches Christ's sacrifice is unrepeatable and once for all (Heb 9:26-28); the Church has always understood the Eucharist not as a new sacrifice but as the sacramental re-presentation of that single, eternal offering, uniting the Church's worship across time to the one Cross.
Biblical foundation
Christ himself instituted the Eucharist as a memorial sacrifice, saying 'do this' — using sacrificial and priestly language — 'in remembrance of me' (Luke 22:19), and Malachi prophesied a 'pure offering' made among the nations 'in every place' (Mal 1:11), fulfilled in the universal Eucharistic offering rather than a new slaying of Christ.
Scripture references
Historical evidence
The Didache (late first century) already calls the Eucharistic gathering a 'sacrifice' that must be pure, fulfilling Malachi's prophecy, showing the sacrificial understanding of the Eucharist long predates medieval theology.
The Church Fathers
“On the Lord's own day gather together and break bread... having first confessed your sins, that your sacrifice may be pure... this is that which was spoken by the Lord: In every place and time offer me a pure sacrifice.”