Why did receiving the previously unrevealed mystery take Paul so long?
The delay was not arbitrary: Paul had to be cleansed of his deep Judaic thinking and prepared to receive a previously unrevealed body of truth. Giving him a clear-cut understanding of things never before revealed naturally required time, as did training him to proclaim it skillfully to Gentiles.
Les explains that people often ask why Saul of Tarsus, after his conversion on the road to Damascus and being led down over the wall from Damascus, would need three years in Arabia. One reason is that Saul was steeped in Judaism and, being just as human as the rest of us, likely needed time to have his thinking and behavior cleansed from Judaism. On the other hand, giving the man such a clear-cut understanding of things that had never been revealed before would naturally take some time. He also had to be so prepared that when he came back from that Arabian experience, ready to go out among the Gentile world and abject Hagan idolaters, he was skilled in what he was proclaiming. Les sees this as part and parcel of the three-year training time down in Arabia. Paul makes it plain that all of this was revealed to him, and he takes the responsibility of passing it out to the whole world.