Ah, Greek… it promises to solve all our interpretations of the New Testament, but instead introduces far more issues. I told this to a group of students learning Greek, and by the look of horror on some of their faces, they hadn't realised it. For all its usefulness, Greek has a tendency to bring up problems which are safely hidden away under the carpet of translation.
Here is one of those issues. Those who are familiar with studies of Paul's theology will realise that one of the most thorny translation issues in Paul's letters is how to render that annoying little duo of words pistis Christou. It could mean 'faith in Christ', as most translations have it, or 'faith(fulness) of Christ' – think of the similarities between 'trust' and 'trustworthy' in English.
Tom Wright, in Paul and the Faithfulness of God, of course, deals with this little expression. Wright is strongly on the 'faithfulness of Christ' side, and explains why, using the occurance of dia pisteōs Iēsou Christou (through the faith/faithfulness in/of Jesus Christ) in Romans 3.22. Wright argues that the strongest reason for the reading 'faithfulness of…' is not gramatical, but rather narrative.
He reasons like this: in 3.2, Paul says that Jews were entrusted (episteuthēsan) with God's oracles. They were meant to be a light to the nations (see 2.17ff). Paul asks, 'does their lack of faithfulness make God unfaithful?' Of course not! Paul, Wright says, is dealing with two problems: (1) all of humanity is sinful; (2) Israel has failed in their commission to be a light to the nations. The question, then, is how is God going to sort out the problem of sin through Israel in order to remain faithful to his covenant.
The answer, Wright explains, is the 'faithfulness of the Messiah,' referred to in 3.22. Christ's faithfulness as the true Israelite means that God can be faithful to his promise to bless the nations through Israel despite their failure to follow their mandate. Pistis Christou, for Wright, is about Christ's faithfulness, as Israel's Messiah, to God's commission. This faithfulness brings about justification.