Why were the believers ready to start preaching to Gentiles at this point in Acts?
God had already begun moving toward Gentiles: He saved Saul, then sent Peter to Cornelius, and now Saul had returned after three years of learning and the revelation of the mysteries. With God's hand with them, Jewish believers who formerly preached only to Jews began speaking to Greeks.
Les explains that the word in this passage should likely be Greeks, not Grecians. A Grecian was a Jew who was not a native of Palestine, so it would not have been unusual for Jewish believers to approach Grecians. A Greek, however, is a Gentile, so speaking to Greeks marked a change in venue and a complete change in their modus operandi. Les ties their readiness to God's prior work: God started the ball rolling toward the Gentile world when He saved Saul, and He speeded it up when He sent Peter to Cornelius. Now Saul had already come back from his three years of learning and the revelation of the mysteries, so they were ready to get the Gospel out to the Gentiles. These Jews who had been preaching to none but Jew only were now beginning to speak to Greeks, preaching the Lord Jesus. The hand of the Lord was with them, and a great number believed and turned unto the Lord.