"And the next Sabbath nearly the whole city assembled to hear the word of God. But when the Jews saw the crowds, they were filled with jealousy, and began contradicting the things spoken by Paul, and were blaspheming. And Paul and Barnabas spoke out boldly and said, 'It was necessary that the word of God should be spoken to you first; since you repudiate it, and judge yourselves unworthy of eternal life, behold, we are turning to the Gentiles. 'For thus the Lord has commanded us,
'I HAVE PLACED YOU AS A LIGHT FOR THE GENTILES,
THAT YOU SHOULD BRING SALVATION TO THE END OF THE EARTH.'
And when the Gentiles heard this, they began rejoicing and glorifying the word of the Lord; and as many as had been appointed to eternal life believed."
In accord with the order set forth by the Lord Jesus Himself, (Acts 1:8), Paul and Barnabas preached the gospel to the Jews first, as was Paul's consistent practice (Romans 1:16). Yet, as so often happened even during the Lord Jesus' ministry, so in this scenario as well, the Jews rejected the Lord Jesus, His claims as Messiah, and especially the thought that Gentiles might have any hope of salvation in Him. Bad enough that they refused to believe for themselves, but to openly contradict Paul & Barnabas' preaching (!), and go so far as to blaspheme... a clear evidence of the impact their proclamation was having, spiritually.
Yet it is when Scripture (Isaiah 42:6) is quoted that a sweeping harvest of souls begins. The Holy Spirit used the passage to cause great rejoicing among the Gentiles in the crowd, "and as many as had been appointed to eternal life believed."
No one believes indiscriminately. No one is saved by happenstance. Belief to salvation is by design, by divine intention, from all eternity. Given the nature of man (Romans 3:10--12), only by such eternal grace could anyone be saved, for no one would seek Him, otherwise. All credit and glory belongs to the triune God for His plan of salvation in grace and mercy!