N. T. is convinced that the traditional construal of πίστις Χριστοῦ in Romans and Galatians as an objective genitive – ‘trust of Christ’ = ‘faith in Christ’ is correct. His reasoning so far, however, is a bit elusive. Mike Aubrey notes a fact about which the “the faithfulness of Christ” people appear to be in denial: the ancient, native-speaker history of interpretation is unanimously behind the objective genitive construal. Thomas of Epistles of Thomas refers to an article which shows that an objective genitive with πίστις is excellent ancient Greek beyond the bounds of Paul. In contrast to those who want to decide the question based on some sort of core meaning of the genitive case in Greek, Carl Conrad notes that “the linkage of the genitive word to its head substantive is to be interpreted depends wholly upon context and how that is interpreted.” We eagerly wait to know why Esteban, no mean linguist himself, favors the objective genitive.
Doug Chaplin establishes the possibility, on theological grounds, that πίστις Χριστοῦ means the faithfulness of Christ, but not its probability. Loren Rosson, as expected, treats us to a very learned discussion of the question. For my money, this is the key graph:
In one of the most careful considerations of Rom 3:21-26 I've come across, Stephen Finlan rightly notes how the subjective genitive makes for a clumsy reading of the passage (The Background and Content of Paul's Cultic Atonement Metaphors, pp 147-148).
To a Hebraist, the traditional interpretation does not seem odd. Objective genitives are not unusual in ancient Hebrew. Relevant examples include the following – for the sake of comparison, I provide the translation into Greek current in the time of Paul:
אָז תָּבִין יִרְאַת יהוה
וְדַעַת אֱלֹהִים תִּמְצָא
τότε συνήσεις φόβον κυρίου
καὶ ἐπίγνωσιν θεοῦ εὑρήσεις
You will learn the fear of יהוה,
you will discover knowledge of God. (Prov 2:5)
וְאֶת־יִרְאָתִי אֶתֵּן בִּלְבָבָם
לְבִלְתִּי סוּר מֵעָלָי
καὶ τὸν φόβον μου δώσω εἰς τὴν καρδίαν αὐτῶν
πρὸς τὸ μὴ ἀποστῆναι αὐτοὺς ἀπ ἐμοῦ
I will put fear of me in their heart
so that they do not turn away from me. (Jer 32:40)
A possible example of אמונה with an objective genitive occurs in 4Q418 81+81a,6:
וטובתי לכה אתן
ואתה הלוא לכה טובי
ובאמונתי הלך תמיד
“And my goodness I will give to you;
you – is not my bounty for you?”
And in trust of me he walked continually.
The context, unfortunately, is broken. The suffixes have been read as third person by some scholars, and הלך, for reasons unknown to me, construed as an imperative. The standard translation, that of Garcia Martinez and Tigchelaar, has "and in faithfulness to him [objective genitive] walk continuously."
I like a few emphases of the "New Perspective," but I'm not interested in pinning very much on newfangled exegesis unless it is very well-supported. Not the case with respect to πίστις Χριστοῦ.