“But this sacred truth must be cleared from an objection whereunto it seems obnoxious, before we do proceed. ‘The will is a natural property, and therefore in the divine essence it is but one. The Father, Son, and Spirit, have not distinct wills. They are one God, and God’s will is one, as being an essential property of his nature; and therefore are there two wills in the one person of Christ, whereas there is but one will in the three persons of the Trinity. How, then, can it be said that the will of the Father and the will of the Son did concur distinctly in the making of this covenant?’
This difficulty may be solved from what hath been already declared; for 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. And whereas all these acts and operations, whether reciprocal or external, are either with a will or from a freedom of will and choice, the will of God in each person, as to the peculiar acts ascribed unto him, is his will therein peculiarly and eminently, though not exclusively to the other persons, by reason of their mutual in-being. 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. And in this respect the covenant whereof we treat differeth from a pure decree; for from these distinct actings of the will of God in the Father and the Son there doth arise a new habitude or relation, which is not natural or necessary unto them, but freely taken on them. And by virtue hereof were all believers saved from the foundation of the world, upon the account of the interposition of the Son of God antecedently unto his exhibition in the flesh; for hence he was esteemed to have done and suffered what he had undertaken so to do, and which, through faith, was imputed unto them that did believe.”
The citation is from Exercitation XXVIII on Federal Transactions between the Father and the Son which is found in volume 2 (pages 87-88).