The KJV translators were the best of the best in their day. The king summoned them to do a translation which would be above all translations. They succeeded in doing this as the KJV Bible is the best English translation ever completed. Obviously, these men were far superior in their study and understanding of Greek and Hebrew languages and of the English language they were translating into than those that have arisen as so called Greek/Hebrew experts in these last days wherein the church has greatly fallen away from truth. A big proof of this is the fact that modern translations are always inferior translations to the KJV.
The KJV translators translated the genitive case in Galatians 2:16 and other like verses as "of Christ" and not as "in Christ".
Galatians 2:16 Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified.
The fact that the little preposition "of" has lead God's elect people into a glorious understanding of the doctrine that we are saved by the faith of Christ and not our own faith--and the fact that this understanding serves to greatly give God all the glory is even more evidence of its trustworthiness.
Just think of the flip side of this coin, those that argue for the translation of "faith in" Christ are arguing for an opening in which man can a little bit of the credit by adding the work of faith. Absolutely not! The so called understanding of the objective genitive (in Christ rather than of Christ) reeks of man's pride trying to take glory from God and give it to himself.