This was written by Isaiah (28:11-12), about the unbelieving Israelites of his own day and age. Yet the same applied to the unbelieving Jews in Jerusalem on Pentecost Sunday. Acts 2:12-15. Indeed, the same also applied to the unbelievers in Corinth –just a few years later. I Cor. 14:23f.
“So then,” continued Paul, “foreign languages [like Assyrian] are a sign not to those who believe, but to the unbelievers!” See Acts 2:4-15. “But prophecy is not for unbelievers, but for those who believe.” Compare I Cor. 14:22a.
Paul was here no doubt thinking of the unbelieving Jews at the time of the Assyrian captivity. Yet he was doubtless also referring here to the unbelieving Jews in Jerusalem on Pentecost Sunday (Acts. 2:13-15) — as well as to the unbelieving Jews in Corinth. Acts 18:1,5,6,12f.