Polhill is a helpful guide in that he unites soteriology with Christology and theology proper. His use of categories is more compatible with the broader Christian tradition, and for that reason, we should feel an imperative to re-read many more of the other older divines:
The sufferings of Christ respect both attributes; they satisfied the law, and founded the gospel. Justice had a full compensation, and mercy sprung up in promises of grace and life…
There was in Christ’s sufferings a conjunction of satisfaction and merit: justice was compensated, and grace impetrated. Indeed the Socinians, blind with their own corrupt reason, cannot see how these two should stand together; satisfaction being the payment of a just debt, and merit the doing of an undue work. To which I answer: it is true, that when one pays a finite sum for his own debt, there is not, there cannot be a merit in it; but when Jesus Christ paid down sufferings of an infinite value for us, there cannot but be an immense merit in them. Infinity is an ocean, and may run over in effects as far as it pleases; those sufferings had a kind of infinity in them, enough to pay divine justice, and over and above by a redundance of merit to purchase all grace for us.
~ A View of Some Divine Truths pg. 5
Note that God’s attributes show who He is and what He is. Justice and Grace are aspects of God:
That God, in all that he doth, acts like himself, in a just decorum to his excellent being and attributes, having no law without or above himself. He conforms to his essence and carries himself so fitly to himself, that no spot, no darkness, no shadow of turning, no indecency or irregularity can possibly happen to him. He cannot deny himself, or do anything unworthy of his being or attributes. He doth whatever he doth, in such a manner as becomes him. Hence Anselm observes, That when God spares and is merciful towards sinners, he is just to himself, and that because he acts condecently to his infinite goodness. This is the first and prime part of his holiness, to be just and true to himself, to do all congruously to his own excellency.
ibid pg. 7
And here Polhill makes the crucial point that justice and mercy are not opposites in God:
God, the righteous rector, who inflicted them [the sufferings of Christ], was one of infinite mercy. Mercy in men, though but finite, is sometimes a remora to punishment. Joseph, being dikaios, a just, that is, (as the word must be taken) a merciful man, would not paradeigmatisai, make Mary a public spectacle of justice. (Matt. i.19.) But though God were one of infinite mercy, and that not merely resident in his nature, but as it were in motion, triumphantly going forth in a most compassionate design towards mankind’ yet he would have justice satisfied in the sufferings of his Son: “To declare, I say, at this time his righteousness,” saith the apostle, (Rom. ii.26.) Observe, it was at this time, it was then a day of salvation, a jubilee of redemption to mankind; yet for all that, justice must have its due, and be declared in the sufferings of Christ. But here the Socinians object, that infinite justice and infinite mercy are opposites, and cannot both be together in God; or if they were, God, who cannot act contrary to anything in his nature, could neither punish because of his mercy, nor yet pardon because of his justice. But I answer, mercy and justice are not opposites in man. After the idolatry of Israel in the molten calf, Moses would in justice have every one slay his brother; yet in a high excess of mercy and charity he would pray, “Forgive their sin; if not; blot me out of thy book.” (Exod. xxxii. 27 and 32.) Neither are they opposites in God, when he proclaims his name in those stately titles; The Lord merciful, gracious, long-suffering, abundant in goodness and truth, keeping mercy for thousands, forgiving iniquity, and transgression, and sin: he yet adds in the close of all, “That he will by no means clear the guilty.” (Exod. xxxiv. 6,7.) Mercy and justice in God have different objects: the penitents, who partake of mercy, are not the objects of justice: the impenitents, who feel justice, are not the objects of mercy. Yet these attributes are not contrary the one to the other; being both divine perfections, they can no more be contrary the one to the other than the divine essence, which both of them are, can be contrary to itself. Cruelty, not justice, is opposite to mercy; injustice, not mercy, is opposite to justice. Neither doth God, in pardoning or punishing, act contrary to any thing in his nature. In pardoning penitents, he acts not against his justice, for that was satisfied in their sponsor Jesus Christ: in punishing impenitents, he acts not against his mercy; for that, as the Socinians themselves confess, extends not to obstinate sinners, neither are they at all capable of it. These two attributes do mutually illustrate one another: the mercy of God is the more illustrious, because when justice was inexorable, it sent his son to suffer for us. The justice of God is the more glorious in Christ’s sufferings, because they were inflicted by one whose mercy was infinite in his nature, and in his design towards men.
ibid pg. 14
Notice too that the climax of the atonement is the death. The merit is in the blood, and this merit is infinite because the blood is God’s blood. This is compatible with what we’ve seen from Rollock, but also (and perhaps more importantly) it works with Anselm and the earlier thought of Augustine and the patristics:
He, the Son of God, very God, assumed our frail nature. But might this infinite and wonderful condescension satisfy justice for the sin of the world? no, he must be under the law, and fulfil all righteousness. Well, that being done, might that obedience (wherein so high an honour was reflected upon th law, as that it was obeyed perfectly in all things, and that by its Maker) satisfy for sin? No, that alone was not enough; there must be shedding of blood, or no remission. But if there must be blood, might not a few drops of his blood, the same being of infinite value, do the work? No, the law calls for death, without that he could not be an expiatory sacrifice.
ibid pg. 16
I fear the folks are using the older terms, but often lack the understanding of their context and use. Thus we often redefine the older terms, and eventually folks react to that redefining with new redefinitions. It doesn’t take long for the whole thing to turn into one big mess.