Acts 18:5-6, Acts 13:45-47, Acts 20:26-27
In the Old Testament, God said that if a man hears the trumpet of God's judgment and does not repent, his blood will return to his head. (Eze 33:2-7)
Eze 33:2 Son of man, speak to the children of thy people, and say unto them, When I bring the sword upon a land, if the people of the land take a man of their coasts, and set him for their watchman: 3 If when he seeth the sword come upon the land, he blow the trumpet, and warn the people; 4 Then whosoever heareth the sound of the trumpet, and taketh not warning; if the sword come, and take him away, his blood shall be upon his own head. 5 He heard the sound of the trumpet, and took not warning; his blood shall be upon him. But he that taketh warning shall deliver his soul. 6 But if the watchman see the sword come, and blow not the trumpet, and the people be not warned; if the sword come, and take any person from among them, he is taken away in his iniquity; but his blood will I require at the watchman's hand. 7 So thou, O son of man, I have set thee a watchman unto the house of Israel; therefore thou shalt hear the word at my mouth, and warn them from me.
Paul testified to the Jews that Jesus was the Christ. But the Jews did not believe and it was their responsibility that they were not saved. (Acts 18:5-6, Acts 20:26-27)
Acts 18:5 And when Silas and Timotheus were come from Macedonia, Paul was pressed in the spirit, and testified to the Jews that Jesus was Christ. 6 And when they opposed themselves, and blasphemed, he shook his raiment, and said unto them, Your blood be upon your own heads; I am clean; from henceforth I will go unto the Gentiles.
Acts 20:26 Wherefore I take you to record this day, that I am pure from the blood of all men. 27 For I have not shunned to declare unto you all the counsel of God.
Because the Jews did not receive the gospel that Jesus was the Christ, this gospel was directed to the Gentiles as prophesied in the Old Testament. (Acts 13:45-47) Acts 13:45 But when the Jews saw the multitudes, they were filled with envy, and spake against those things which were spoken by Paul, contradicting and blaspheming. 46 Then Paul and Barnabas waxed bold, and said, It was necessary that the word of God should first have been spoken to you: but seeing ye put it from you, and judge yourselves unworthy of everlasting life, lo, we turn to the Gentiles. 47 For so hath the Lord commanded us, saying, I have set thee to be a light of the Gentiles, that thou shouldest be for salvation unto the ends of the earth.