The Covenant of Redemption
In Reformed theology, the pactum salutis has been defined as a pretemporal, intratrinitarian agreement between the Father and Son in which the Father promises to redeem an elect people. In turn the Son volunteers to earn the salvation of his people by becoming incarnate...by acting as surety of the covenant of grace for and as mediator of the covenant of grace to the elect. In his active and passive obedience, Christ fulfills the conditions of the pactum salutis...ratifying the Father's promise, because of which the Father rewards the Son's obedience with the salvation of the elect. And because of this the Holy Spirit applies the Son's work to his people through the means of grace.
David Van Drunen & Scott Clark: Covenant, Justification and Pastoral Ministry, p. 168