27 “My mercy will I keep for Him for ever: and my Testament faithful with Him”. On His account, the Testament is faithful: in Him the Testament is mediated: He is the Sealer, the Mediator of the Testament, the Surety of the Testament, the Witness of the Testament, the Heritage of the Testament, the Coheir of the Testament.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)