But the grammar doesn't allow it!

I suggest that a better solution to the πίστις Χριστοῦ debate lies somewhere in the middle between “faith in Christ” and the “faithfulness of Christ.” I find the traditional “faith in Christ” position, though rich in heritage and safeguarding the call for a human response to the gospel, ultimately dissatisfying because it does not capture the participationist themes that Paul weaves into his theological discourse. However, the “faithfulness of Christ” option, overflowing with a vat of theological new wine, doesn’t work for those of us damned with too much knowledge of Greek grammar, and it seems like a theological overread.—Michael Bird, An Anomalous Jew: Paul among Jews, Greeks, and Romans, 143