A direct quote is not intertextuality, in my humble open. It is a quote. It is not an allusion, because it not hidden. It is a quote. However, in studying Romans, the exegete must have studied for some time the books of Deuteronomy and Isaiah. Found this and thought it would be useful…
Paul not only quoted from Isaiah many more times than all of the other prophets put together, but he actually used the prophet’s writings as the skeleton of his gospel. He took the quotations and arranged them in such a way as to outline the history of salvation, from the Fall of man to the eventual establishment of the messianic kingdom. Around these quotations he built his argument. The full import of this fact is only appreciated when the quotations are listed in the order they are used and read in that same sequence. What it shows is that if the letter is laid out as a continuous papyrus, and the citations from Isaiah were raised out of the text and suspended at their point of use, those texts, in that order, summarise the whole of salvation history. Such a pattern could not be anything but intentional.
The cited texts of Romans.
We follow the Isaianic texts Paul cited in the order that he used them:
“As it is written ‘God’s name is blasphemed among the Gentiles because of you’” (Romans 2:24: Isaiah 52:5, LXX).
“Their feet are swift to she d blood: ruin and misery mark their paths and the way of peace they have not known”(Romans 3:15-17: Isaiah 59:7-8).
“Isaiah cries out concerning Israel: ‘Though the number of the Israelites should be like the sand by the sea, only the remnant will be saved. For the Lord will carry out his sentence on earth with speed and finality’. It is just as Isaiah had said previously” (Romans 9:27-28:Isaiah 10:22-23, LXX).
“Just as Isaiah said previously, ‘Unless the Lord Almighty had left us descendants, we would have become like Sodom, and we would have been like Gomorrah’” (Romans 9:29;Isaiah 1:9, LXX).
“As it is written,’ See, I lay in Zion a stone that causes men to stumble and a rock that makes them fall’” (Romans 9:33a; Isaiah 8:14).
“and ‘the one who trusts in him will never be put to shame’” (Romans 9:33; Isaiah 28:16, LXX).
“As the Scripture says. ‘He who believes in him will not be disappointed’” (Romans 10:11;Isaiah 52:7, LXX).
“As it is written, ‘How beautiful are the feet of those who bring good news!’” (Romans 10:15; Isaiah 52:7).
“For Isaiah says, ‘Lord, who has believed our message’”? (Romans 10.16; Isaiah 53:1,LXX).
“And Isaiah boldly says, ‘I was found by those who did not seek me, I revealed myself to those who did not ask for me’” (Romans 10:20; Isaiah 29:10, LXX).
“What then? What Israel sought so earnestly it did not attain, but the elect did. The others were hardened as it is written: ‘God gave them a spirit of stupor, eyes so that they could not see and ears so that they could not hear, to this very day’” ( Romans 11:7-8; Isaiah 29:10).
“And so all Israel shall be saved: as it is written, ‘there shall come out of Zion the Deliverer, and shall turn away ungodliness from Jacob; for this is my covenant unto them when I shall take away their sins’” (Romans 11:26-27; Isaiah 59:20-21, LXX).
“Who has known the mind of the Lord? Or who has been his counsellor” (Romans 11:34;Isaiah 40:13, LXX).
“For it is written, ‘As I live, sayeth the Lord’” ( Romans 14:11a; Isaiah 49:18).
“Every knee shall bow to me, and every tongue shall confess to God” (Romans 14:11b;Isaiah 45:23, LXX).
“And again, Isaiah says, ‘The root of Jesse will spring up, one who will arise to rule over the nations; the Gentiles will hope in him’” (Romans 15:12; Isaiah 11:10, LXX).
“Rather, as it is written, ‘Those who were not told about him will see, and those who have not heard will understand’” (Romans 15:21; Isaiah 52:15, LXX).
These passages show the perspective which Paul had in regard to salvation history. It was that of the evangelical prophet. The quotations work systematically through the various stages of the development of the purposes of God in the salvation of Mankind.
Israel has not responded to her calling, she has acted like the other nations (Romans 2:24;Isaiah 52:5: Romans 3:15-17; Isaiah 59:7-8).
God’s purpose is to show his faithfulness to his promises by saving a remnant. (Romans 9:27-29; Isaiah 10:22-23).
God will appoint a saviour, for both Jews and Gentiles (Romans 9:33; Isaiah 8:14; 28:16:Romans 10:11: Isaiah 8:16). Notice how Paul stresses the universality of Christ’s salvation as he follows up the quotation of Isaiah 28:16 with: “For there is no difference between Jew and Gentile – the same Lord is Lord of all and richly blesses all who call on him, for ‘Everyone who calls on the name of the Lord will be saved’”.
Paul then goes on to speak of the church’s responsibility to declare the salvation of God, as it had been fulfilled by the remnant in the previous age ( Romans 10:15; Isaiah 52:7).
But there would be the same response of unbelief to the gospel message ( Romans 10:16;Isaiah 53:1).
Even so, the electing purposes of God would not be overturned by the sinfulness of Man. What he purposes he will achieve (Romans 10:22; Isaiah 65:1; Romans 10:21; Isaiah 65:2;Romans 11:8; Isaiah 29:10).
God’s purposes will be fulfilled, and all Israel, as Paul has already defined her (Romans 4:11-12), will be saved (Romans 11:26-27; Isaiah 59:20-21).
All of this is beyond man’s design, it is of God alone (Romans 11:33-34; Isaiah 40:13).
The salvation promised to Abraham, in which the nations are to share in the covenant blessings, will finally be fulfilled. Those who were never part of the people of God have come into the eschatological community. (Romans 15:21; Isaiah 52:15)