en_tn_condensed/heb/09/15.md

he is the mediator of a new covenant

See how you translated a similar phrase in Hebrews 8:6-7.

to redeem those under the first covenant from their transgressions

"to take away the transgressions of those who were under the first covenant." or "to take away the guilt of those who were under the first covenant." or "to take away the punishment that those who were under the first covenant deserved because they had transgressed"

those who are called

"those whom God has called" or "those whom God has chosen to be his children"