He said is literally “saying.” Good News Translation divides the Greek sentence, which runs without a break from verse 18 to verse 21. “As he did this, he said” would be possible. The quotation is from Exodus 24.8, with some changes, probably influenced by the story of the Last Supper (Mark 14.24).

The reference of This may need to be more explicit; for example, “What I am sprinkling.”

Which seals is implied and brings out the meaning of “blood of the covenant.” The verb seals is a dead or dying metaphor in English, since the practice of fastening documents with a seal has become rare. It would be possible to bring out the meaning in a nonfigurative way, for example, by translating “This is the blood which confirms the covenant to which God has ordered you to be faithful.”

It is unusual to speak of one party to an agreement commanding the other to obey it. But the biblical covenant was unique; see the comment on 7.22. This idea is seriously weakened by Die Bibel im heutigen Deutsch‘s “Through this blood is sealed the covenant which God has concluded with you.”

In some languages “to seal a covenant” or “to confirm a covenant” may be expressed as “to make a covenant strong.” But the concept is often expressed somewhat more idiomatically; for example, “it ties you to the covenant” or “it makes you one with the covenant.”

To obey is implied in commanded.

The clause that God has commanded you to obey may have to be expressed as direct discourse; for example, “God has commanded, ‘You must obey the covenant’ ” or “ ‘… obey what the covenant says you must do.’ ”

Quoted with permission from Ellingworth, Paul and Nida, Eugene A. A Handbook on The Letter of the Hebrews. (UBS Handbook Series). New York: UBS, 1983. For this and other handbooks for translators see here .