From John L. Girardeau, The Federal Theology: Its Import and Its Regulative Influence, 7-9:
There would seem to be no necessity to distinguish, as some have done, between the covenant of redemption and the covenant of grace as two separate covenants: the former as conceived to exist between God the Father and Christ, and the latter between God and the elect. For, in the first place, the law of parcimony opposes the supposition of two covenants. This presumption could only be removed by such explicit testimony of Scripture to the existence of two as can hardly be contended for in the face of another construction of its teaching by so many theologians.
In the second place, it is inconceivable that God would have entered into a covenant with sinners except in Christ as Mediator and Federal Head. To say that one covenant was made with the Son and another with the elect, is to assume as the differentia of the latter the fact that it was not made with them in Christ, but apart from him. But that cannot be admitted. To reply that the covenant, though not made with him, was made with the elect as in him, is to give up the distinction. The covenant, according to the ordinary conception and statement of it, was at the same time made with him and with his elect seed in him. It is wholly unwarrantable to hold that a federal arrangement should obtain in relation to sinners, except as they are represented by a federal head. The covenant with Christ, therefore, embraced the covenant with his elect constituency. They are never dealt with except as they are in him.
In the third place, let it be conceded that the covenant wears two aspects, one immediately contemplating Christ as federal head and representative, and the other, the elect as beneficiaries, and they are evinced to be but separate faces of the same great compact by the consideration that the privileges, graces, and duties of the elect are benefits conferred upon them in Christ, are but parts of that salvation which he meritoriously secured for them by his perfect performance of covenanted righteousness. Their faith, it is true, as an indispensable duty, conditions their subjective and conscious union to Christ, but faith is the necessary result of regeneration, in which they are the passive recipients of the grace acquired for them by their federal head. That which is held to be a covenant of grace, in distinction from the covenant of redemption, may be regarded as but a testamentary administration, in behalf of the elect, of the one eternal covenant between the Father and the Son.
It may be added, in the fourth place, that the analogy between the covenant of grace and that of works, which is universally admitted to have been but one, and the language of the Calvinistic symbols which must be strained to support any other supposition, oppose strong presumptive evidence to the hypothesis of two distinct covenants. It is one and the same covenant, which, regarded in relation to the means employed and the end contemplated, is denominated the covenant of redemption, that is emphatically designated the covenant of grace when conceived in reference to its source, and to its unmerited application to sinners as the recipients of its benefits. It is peculiarly a covenant of grace to them, since its legal condition was fulfilled, not by themselves, but by another for them, guilty and corrupt.
Reformed Theology