Why did Paul need to be so thoroughly prepared during his time in Arabia?
Paul needed thorough preparation because he had been steeped in Judaism and had to have his thinking cleansed. The mysteries he would proclaim had never before been revealed, so understanding them took time. He also had to be skilled before going among pagan Gentiles, and his three years in Arabia provided that training.
Les explains that people have often asked why Saul of Tarsus, after his conversion on the road to Damascus, was led down over the wall and taken by God to Mount Sinai in Arabia for three years. One reason is that Saul was steeped in Judaism and, being just as human as anyone else, probably needed time to cleanse his thinking and behavior under Judaism. On the other hand, giving him a clear-cut understanding of things that had never been revealed before would naturally take time. He had to be so prepared that when he returned from that Arabian experience and was ready to go out among the Gentile world and abject heathen idolaters, he would be skilled in what he was proclaiming. Les sees this as part and parcel of that three-year training time down in Arabia. Yet Paul makes plain that all of it was revealed to him, and he took the responsibility of passing it out to the whole world.