“this is my blood.” Here in Matthew, Jesus emphasizes that his death paid the penalty for Israel breaking the Old Covenant (the Mosaic Covenant). In Luke, Jesus emphasizes that his death ratifies the New Covenant.
[For more information on the difference between Matthew and Luke, and the two aspects of Jesus’ death, see commentary on Luke 22:20.]
“covenant.” See commentary on Hebrews 7:22. The word “new” is not included, as it is in the KJV. Textual scholars conclude that it was added to some Greek texts so that this verse mirrored Luke 22:20 more closely. Had “new” been original, there is no good reason for dropping it from the early texts.