Many Reformed theologians have taught that redemption in history unfolds from a “covenant of redemption” (pactum salutis) between the Father and Son. As Francis Turretin defined it,
the pact between the Father and the Son contains the will of the Father giving the Son as lytroten (Redeemer and head of his mystical body) and the will of the Son offering himself as sponsor for his members to work out redemption. . . . For thus the Scriptures represent to us the Father in the economy of salvation as stipulating the obedience of the Son even unto death, and for it promising in return a name above every name that he might be head of the elect in glory; the Son as offering himself to do the Father’s will, promising a faithful and constant performance of the duty required of him and restipulating the kingdom and glory promised him” (quoted in an essay by Scott Swain in Christian Dogmatics, 117).
One of the criticisms brought against this notion is that it is implicitly tritheist. Barth charges that it requires us to think of Father and Son “as two distinct subjects and therefore as two legal subjects who can have dealings and enter into obligations with one another” (quoted by Swain, 121). Differently stated, the objection is that God has only a single will, but the pactum salutis suggests a diversity of wills.
In response to this objection, Swain quotes a John Owen and Wilhelm a Brakel, who acknowledged the difficulty and responded with statements about the personal differentiation of the one Triune will.
Such is the distinction of the persons in the unity of the divine essence, as that they act in natural and essential acts reciprocally one towards another—namely, in understanding, love, and the like; they know and mutually love each other. And as they subsist distinctly, so they also act distinctly in those works which are of external operation. . . . The will of God as to the peculiar actings of the Father in this matter is the will of the Father, and the will of God with regard unto the peculiar actings of the Son is the will of the Son; not by a distinction of sundry wills, but by the distinct application of the same will unto its distinct acts in the persons of the Father and the Son (121–2).
And a Brakel:
Since the Father and the Son are one in essence and thus have one will and one objective, how can there possibly be a covenant transaction between the two, as such a transaction requires the mutual involvement of two wills? Are we then not separating the persons of the Godhead too much? To this I reply that as far as personhood is concerned the Father is not the Son and the Son is not the Father. From this consideration the one divine will can be viewed from a twofold perspective. It is the Father’s will to redeem by the agency of the second person as surety, and it is the will of the Son to redeem by his own agency as surety (122).
Leave aside the question of the pactum salutis itself. Swain captures the essential implication for Trinitarian theology: The pactum salutis forces us to acknowledge not only the unity of God’s will, but to see “that will’s tripersonal manner of subsistence” (122).