Comparing the NIV & KJV in Three Instances the Greek Genitive is Used

Amazing how every time the N.T. mentions the 'faith of Christ' it seems to be an objective genitive according to many modern scholars (like those that translated the NIV).

However, the NIV translators seem to translate the genitive case properly when it does not involve Christ's faith. For example, this is taken from the NIV:

Galatians 6:2 Carry each other’s burdens, and in this way you will fulfill the law of Christ....

"Fulfill the law OF Christ". The KJV translates this verse the same way.

I'm looking over the Greek used in Galatians 6:2 that the translators translated as 'of' and I just can't see the difference between this and the Greek used in Galatians 2:16 or in Galatians 3:22?

Here's the three verses in the Greek:

Galatians 2:16 ...διὰ πίστεως Χριστοῦ Ἰησοῦ

Translated as:

...by the faith of Christ (KJV) ...by faith in Jesus Christ (NIV)

Galatians 3:22 ...ἐκ πίστεως Ἰησοῦ Χριστοῦ

Translated as:

...by faith of Jesus Christ (KJV) ...through faith in Jesus Christ (NIV)

Galatians 6:2 ...ἀναπληρώσετε τὸν νόμον τοῦ Χριστοῦ

Translated as:

...fulfil the law of Christ (KJV) ...fulfill the law of Christ (NIV)

The NIV translates the genitive case as "of" in this verse. But why? The KJV translators are consistently translating the genitive case time after time. But the NIV translators seem to always translate the genitive as 'in' whenever faith and Christ are involved.

There does not seem to be a grammatical rule being applied, but rather a theological rule. A doctrinal rule.

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