Hans Küng's Justification: The Doctrine of Karl Barth and a Catholic Reflection proposes a solution to the articulus stantis et cadentis ecclesiae that has captivated me, and I've been writing a series on this excellent work.
In the following quotations, Küng addresses the question of whether the Roman Catholic Doctrine of Justification (and Trent too) is synergistic, or as Küng calls it: Partly-Partly. And his answer is No! The difference between Catholics and Protestants is a matter of emphasis. Protestants focus on the work of God in justifying the man, and Catholics focus on the outcome of the justification of a man. Both are entirely the work of God. The Protestants do not deny that the justification of man by God is without results in the sinner, and that the Catholics do not deny that the works of a man are entirely given to the man who works. Protestants and Catholics emphasize the different sides of the same coin of the Doctrine of Justification.
There remains a further discussion to be had over what Protestants and Catholics mean by grace (habitus) that is needed to answer all the questions, but the following quotations explains how Protestants and Catholics are arguing about two sides of the same coin.
The first quotation contains the assertion that the differences between Protestants and Catholics are only imaginary difference:
Trent’s teaching on justification can be correctly understood only in the context of history of dogma. In this context, however, it can and must be understood correctly. This, for the time being, is our preliminary answer to Karl Barth’s polemic against Trent. Protestants speak of a declaration of justice and Catholics of a making just. But Protestants speak of a declaring just which includes a making just; and Catholics of a making just which supposes a declaring just. Is it not time to stop arguing about imaginary differences?
Küng, Hans Justification: The Doctrine of Karl Barth and a Catholic Reflection. Louisville, KY: Westminster John Knox, 2004. 221. Print.
This next quotation rejects the Protestant criticism that the Roman Catholic Doctrine of Justification is partly-partly, such that man works in part and God works in party, in a "synergism in which God and man pull on the same rope." Küng demonstrates that no such synergism belongs to the Roman Catholicism.
Kirchgaessner: “‘Be reconciled with God!’ This means man must respond to the redemptive will of God so as to have his mind changed in regard to God. Faith and conversion is thus the second step, made necessary as soon as God has taken the first. At first man is passive. God acts on him as an object, but it is precisely through this activity of God that he is made active” (Erlösung p. 105)
Therefore, Trent’s cooperari implies no synergism in which God and man pull on the same rope. It is never as though justification came partly from God and partly from man. It has been sufficiently emphasized that the sinner can do nothing without the grace of Jesus Christ. “What have you that you did not receive?” (1 Cor 4:7). “For God is at work in you, both to will and to work for his good pleasure” (Phil 2:13). “Not that we are sufficient of ourselves to claim anything as coming from us; our sufficiency is from God” (2 Cor 3:5).
Everything comes from God, even what man does. A “supplementation” of divine justification is out of the question, God’s glory is not belittled. God wants man’s highest activity, but this can grow only from a complete passivity, from a receptivity brought about by God. The vital point is that God accomplishes everything. But it does not follow from this that He accomplishes it alone. On the contrary the greatest marvel of God’s accomplishing everything is that man accomplishes along with Him as a result of God’s accomplishment. Sacred Scripture makes both points, as does Trent (D 797): “Restore us to thyself, O LORD, that we may be restored!” (Lam 5:21), and “Return to me, says the LORD of hosts, and I will return to you” (Zech 1:3)
And an additional response explaining the same thing in response to Karl Barth:
We have seen once again how unfounded was [Karl] Barth’s polemic against the Council of Trent. It may have become clear now that, according to Catholic and Tridentine teaching on justification too, there is no other recourse for the sinner than to place his whole trust in the Lord.
In the problem of the certainty of faith, Barth’s misunderstanding of the strictly “subjective” character of the Tridentine concept of justification shows up once again. Obviously, Trent did not intend to question the certainty of, and absolute confidence in, that (“objective”) justification which took place for everything in the death and resurrection of Christ. But in the question of certainty as to the (“subjective”) realization of justification, and in the matter of trust in this having happened, the Council intended to make sure that its approach was tempered by an awareness of human frailty and sinful unreliability.
But this discovery is not enough for an answer to our questions. We need further clarification. What about human co-operation, sanctification, and merit?
Much of the differences between Protestants and Catholics is how to understand the word "grace" or habitus.
Barth's fears that God's grace might become, perniciously, 'my' grace are unfounded if we keep in view the fact that grace is mine only as the grace of God; I never "have" it; it is never simply at my disposal. The term habitus is not meant in the sense of "having" grace, but, as Bonaventure explains "to hold is to be held" [..]. Grace is given to me each day as something completely new. It becomes "my" grace--as a consequence of the incarnation--but always as a grace alien to me, according to the paradoxical formulation of Trent: [..] ("Thus, it is not personal effort that makes justice our own."--D809). The 'Index of Celestine' states in Chap. 2: "Unless he who alone is good grants a participation in his being, no one has goodness within himself. This truth is proclaimed by that prontiff (Innocent I) in the following sentence: 'For the future, can we expect anything good from those who mentality is such that they think they are the cause of their goodness and do not take into account him whose grace they obtain each day, and who hope to accomplish so much without him?" And in Chap. 6: "The same teacher Zosimus instructed us to acknowledge this truth when, speaking to the bishops of the world about the assistance of divine grace, he said: 'Is there ever a time when we do not need his help? Therefore, in every action and situation, in every thought and movement, we must pray to him as our helper and protector'" (D 131 and 135)