It's not that out of reach. So there are Judaizers you see and they want to wring Paul's neck. Are you really going to say that you are not interested? Why does he quote the Isaiah passage to them at the end of the sermon in 13? Why does he say in the hearing in ch 26 that the thing which Israel wants to see fulfilled is fulfilled already in the resurrection? And you are not interested? He says that risking his life!
As for whether it has a bearing on the Gospel...well, it is the Gospel. The event of Christ dying for our sins and justifying us through that event is the Gospel, so there is some kind of vacuum in what you are saying and you need to find out what it is filled with. "I determined to preach nothing except Christ crucified among you"--sounds pretty major to me!
He quotes the Isaiah passage because he is reminding them of what God had called them unto that He has concluded them unfit towards, seeing as, they have judged themselves unworthy of the eternal life that would have been theirs on His return.
Matthew 10:5 These twelve Jesus sent forth, and commanded them, saying, Go not into the way of the Gentiles, and into any city of the Samaritans enter ye not: 10:6 But go rather to the lost sheep of the house of Israel.
Matthew 10:19 But when they deliver you up, take no thought how or what ye shall speak: for it shall be given you in that same hour what ye shall speak. 10:20 For it is not ye that speak, but the Spirit of your Father which speaketh in you. 10:23 But when they persecute you in this city, flee ye into another: for verily I say unto you, Ye shall not have gone over the cities of Israel, till the Son of man be come.
Paul is NOT quoting Isaiah to Israel in Acts 13 as some sort of a statement that he is following THEIR commission - that is YOUR error.
He is pointing out their failure to receive Acts 3's preaching to THEM and its OFFER to blot out of THEIR sin at the Lord's return, which is when the GENTILE aspect of THEIR commission would THEN kick in - AFTER THEIR - ISRAEL'S salvation FIRST.
He is telling them that God has turned from THEIR nation as to THEIR commission and ITS' order.
That He HAS TURNED to the GENTILES not only NOW, but DIRECTLY, rather than THROUGH Israel FIRST.
Paul's is NOT their SAME commission.
THEIRS is YOU FIRST - YOUR rise FIRST - and THEN the Gentiles.
Isaiah 60:1 Arise, shine; for thy light is come, and the glory of the LORD is risen upon thee.
When - AFTER His return...
Isaiah 60:2 For, behold, the darkness shall cover the earth, and gross darkness the people: but the LORD shall arise upon thee, and his glory shall be seen upon thee. 60:3 And the Gentiles shall come to thy light, and kings to the brightness
of thy rising.
Isaiah 62:1 For Zion's sake will I not hold my peace, and for Jerusalem's sake I will not rest, until the righteousness thereof go forth as brightness, and the salvation thereof as a lamp that burneth. 62:2 And the Gentiles shall see thy righteousness, and all kings thy glory: and thou shalt be called by a new name, which the mouth of the LORD shall name.
In CONTRAST to that, Paul's is an enigma, a seeming riddle or a mystery - the Gentiles' salvation DURING ISRAEL'S Fall.
Romans 9:30 What shall we say then? That the Gentiles, which followed not after righteousness, have attained to righteousness, even the righteousness which is of faith. 9:31 But Israel, which followed after the law of righteousness, hath not attained to the law of righteousness. 9:32 Wherefore? Because they sought it not by faith, but as it were by the works of the law. For they stumbled at that stumblingstone; 9:33 As it is written, Behold, I lay in Sion a stumblingstone and rock of offence: and whosoever believeth on him shall not be ashamed.
Romans 11:11 I say then, Have they stumbled that they should fall? God forbid: but rather through their fall salvation is come unto the Gentiles, for to provoke them to jealousy. 11:12 Now if the fall of them be the riches of the world, and the diminishing of them the riches of the Gentiles;
how much more their fulness?
Romans 11:25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in. 11:26 And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob: 11:27 For this is my covenant unto them, when I shall take away their sins. 11:28 As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers' sakes. 11:29 For the gifts and calling of God are without repentance.