Why couldn't Paul have written that the Word of God was committed to Israel if Luke was a Gentile?
Because Luke authored the Gospel of Luke and Acts, a substantial portion of the New Testament. If Luke were a Gentile, then part of the Word of God would have been committed to a Gentile, contradicting Paul's Romans 2 declaration that Israel was advantaged because unto them were committed the Word of God.
Les explains that Paul is often castigated, rejected, and ridiculed, yet he remains the major writer of the New Testament. He also notes that an article substantiated what he had always claimed: Luke was a Jew. His basis is Paul's statement that Israel was more advantageous than any other group because unto them were committed the Word of God. Since Luke wrote both the Gospel of Luke and Acts, a goodly portion of the New Testament, if Luke were a Gentile then Paul could not have rightly written by inspiration in Romans 2, "what profit then hath the circumcision, much every way, because unto them were committed the Word of God." So the article confirmed this and thrilled him.