Paul is the single most important apostle to the gentiles. Without him, I think Chrisitanity would have disappeared into history as a strange sect of judaism like the essenes. God chose Paul because I think Paul could relate to Romans and greeks better being more hellenized Jew compared to the 11 poor fisherman of Palestine. Think of Acts or in Galatians where Paul confronted Peter and others of a jew focused ministry the 11 were backsliding into. He was the cultural bridge that could better relate the gospel to the gentiles.
I don't think it's that cut and dried, unless the discussion discounts the Holy Spirit.
Acts clearly shows us that the Holy Spirit had begun using Hellenists out of Antioch as a powerful evangelical force before Paul got involved.
Paul had been himself the chief and most violent "Judaizer" among the Hellenists (the "Judaizers" were Hellenist Jews, not Hebraic Jews). Merely getting him off that particular team was a great boon to the Hellenist evangelists, even if Paul had not become one of them.
But if, say, Paul had decided to stay holed up in Tarsus instead of going to Macedonia, I don't think God would have been any more dependent on him that God was dependent on Elijah.
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