Turning to the Gentiles, Paul and Barnabas quoted the Lord: 'I have made you a ___ for the Gentiles.'

Answer: Light

At Pisidian Antioch they quoted: "I have made you a light for the Gentiles, to bring salvation to the ends of the earth."

“For this is what the Lord has commanded us: ‘I have made you a light for the Gentiles, to bring salvation to the ends of the earth.’””— Acts 13:47 (BSB)