Prov 16.6 says: By loyalty and faithfulness iniquity is atoned for.
This is a plausible line of thought in its own right. How can a person make good for a deed done wrong? How can the relationship be restored if there has been a misstep? It is obvious that trust has to be reestablished, especially in the case of grave wrongdoings; the most obvious way for this to take place is for the other party to demonstrate goodwill and a concern for the health of the relationship through loyalty and faithfulness. This may include apology, reparations, and the rest.
Understanding the point, then, it seems that this insight can be used fruitfully in our atonement theology as well. What is it about Christ's work that atones for our sins? What is it that makes right the wrong humanity has done, and restores the relationship between God and human persons?
It seems to me we ought to make room for Christ's faithfulness to God, in other words his general obedience as a human, in our conceptualization of the atonement. Human sin is atoned for when the representative of humanity, Jesus of Nazareth, takes the burden of God's law upon himself and obeys it for the rest. As I understand it, this is what happens when Christ participates in the baptism of repentance called for by John the Baptist. He is taking the burden of the sin of Israel and humanity upon himself, and being obedient and righteous in their place for their sake.
This means that we ought not limit our understanding of the atonement to the fact of Christ's death. Certainly it plays an important role, and I find Christ's willingness to undergo death on behalf of others as a part of his fulfillment of the law to "love thy neighbor as thyself." But atonement is something that takes place throughout the whole life of Christ, as he repairs the relationship between God and humanity within his own person, through his own righteousness and obedience.
Here, it seems to me, are good grounds for understanding the πίστις Χριστοῦ genitives in Paul subjectively, as references to the faithfulness of Christ. If atonement requires fidelity and loyalty, then it makes sense that Paul would connect atonement, redemption, etc., to Christ's fidelity to God. When he says, for instance, that God προέθετο [τὸν Ἰησοῦν] ἱλαστήριον διὰ πίστεως ἐν τῷ αὐτοῦ αἵματι (Rom 3.25), we may understand him following Campbell in the following way: an atonement for sin, through his faithfulness, by his blood. The atonement is realized through Christ's faithfulness unto death, metonymically referred to as "his blood."
This has important consequences for our own practices of atonement, too. Atonement as I understand it is not about making the other person pay for what they've done, but about establishing and evidencing the necessary goodwill for a relationship to move forward. You can't be my friend if you don't sense that I have goodwill for you and am disposed to act on it in our interactions. Atonement is about my demonstrating that goodwill to you in a way that you can appreciate and (I would hope) is proportionate to the gravity of the wrongdoing.