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 (the Spirit having prepared a body for him), 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 and fulfills his guarantee … 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 VanDrunen & R. Scott Clark, Covenant, Justification and Pastoral Ministry: Essays by the Faculty of Westminster Seminary California, (Phillipsburg, NJ:Presbyterian and Reformed, 2007), 168.
I highly recommend this book available here: Covenant, Justification and Pastoral Ministry: Essays by the Faculty of Westminster Seminary California