Acts 28:28 Be it known therefore unto you, that the salvation of God is sent unto the Gentiles, and that they will hear it.
The Jews had now rejected the gospel. They had turned away from Christ and His teaching. Now Paul would go to the Gentiles. He knew that as he preached to them that they would hear the gospel.
The Jews had too many built-in prejudices to accept what Jesus preached. They wanted it to be their way. They refused to believe the truth even when clearly presented. The truth would mean that they had been wrong on so much for so long.
We can go preaching knowing that they will listen. We can have confidence that those that have never heard receive the truth better than those that know so much that they proclaim themselves ignorant.