The Jews rejected the message, but God had ordained that the Gentiles would also hear the message. The Jew first, then the Gentile. That’s the pattern God put in place.
God doesn't ordain any Gentile to eternal life until he has followed his conscience and is willing to hear the Word of God (vs. 42,48) "ordained" something God allows to come to pass.
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.
48 And when the Gentiles heard this, they were glad, and glorified the word of the Lord: and as many as were ordained to eternal life believed.
We should note that the word "ordain” never fixes anything absolutely. (Lk. 7:8; Rom. 13:1; Acts 28:23, Matt. 28:16; 1 Cor. 9:14; 7:17) Paul ordained some things and that no more meant they were absolutely fixed to come to pass than if he hadn't used the word. (Rom. 13:1) The meaning is obviously that as many as were disposed to believe the message, believed.