In these few verses Paul quotes a lot of Scripture.
Romans 15:1–13
1 We who are strong have an obligation to bear with the failings of the weak, and not to please ourselves. 2 Let each of us please his neighbor for his good, to build him up. 3 For Christ did not please himself, but as it is written, “The reproaches of those who reproached you fell on me.” 4 For whatever was written in former days was written for our instruction, that through endurance and through the encouragement of the Scriptures we might have hope. 5 May the God of endurance and encouragement grant you to live in such harmony with one another, in accord with Christ Jesus, 6 that together you may with one voice glorify the God and Father of our Lord Jesus Christ.
7 Therefore welcome one another as Christ has welcomed you, for the glory of God. 8 For I tell you that Christ became a servant to the circumcised to show God’s truthfulness, in order to confirm the promises given to the patriarchs, 9 and in order that the Gentiles might glorify God for his mercy. As it is written, “Therefore I will praise you among the Gentiles, and sing to your name.” 10 And again it is said, “Rejoice, O Gentiles, with his people.” 11 And again, “Praise the Lord, all you Gentiles, and let all the peoples extol him.” 12 And again Isaiah says, “The root of Jesse will come, even he who arises to rule the Gentiles; in him will the Gentiles hope.” 13 May the God of hope fill you with all joy and peace in believing, so that by the power of the Holy Spirit you may abound in hope.
Let's work backwards through these references.
In verses 9-12 Paul looks back at the words of the prophets in the OT Scriptures and see this has always been the plan. It's almost as if he has this "Aha!" moment and realizes what has been there but he has been overlooking all along.
He points to 2 Sam. 22:50 (also found in Ps. 18:49), Deut. 32:43 (This version Paul cites is the Septuagint version as seen in the NASB), Ps. 117:1, and finally Is. 11:10. All of these texts indicate that it has always been God's plan to include the Gentiles (all the nations) into his people.
Yesterday we looked at the reference in v. 3 to Ps. 69. In verse 4 Paul says something that we could easily overlook. Remember he is writing to a group of house churches consisting of Jews and Gentiles. Previously the Scriptures would have only been thought of as written for the Jews but here he says they are for our instruction. The Scriptures are now for the instruction of all peoples. They should all alike look to the Scriptures and through the endurance and encouragement there find hope.
Here's what he means by that. In the opening act of God's redemption plan he calls Abraham and tells him that he will be a blessing to all the families of the earth (Gen. 12:3). Then for centuries God focuses on one group of people—the Israelites. Generation after generation go by and they see glimpses of this promise being fulfilled and their hope being vindicated. Characters like Joseph, David and Daniel seem like great candidates but all of them fall short of truly blessing all the families of the earth. Then, at long last, after all these long years, when many had lost hope, Jesus comes on the scene and fulfills this promise confirming that God's Word is good. He will fulfill what he promises to do. Because of this, Paul and his readers can have a confident assurance that these groups with serious disagreements will but united.
Additional Content
The Bible Project guys do a helpful breakdown of the big story of Scripture and the theme of the redemption of all the nations.
Reflection
For reflection time today think of the big story of Scripture. What would it have been like to live in the time of David, roughly 900 BC. Waiting, hoping that he would be the Messiah, the one to fulfill God's promise to Abraham. Then think of the disappointment when you realize that it wasn't him. Think of what it must have been like to live in the exile during the time of Daniel. First you're in exile, how can the Messiah possibly come as a blessing to the nations there. Think of the hopelessness the first century Jews must have been experiencing waiting for God's promise to come true. In the midst of some of the more hopeless situations in the church we would do well to remember that through endurance and the encouragement we find in Scripture there is always hope. God's promises have never failed and his promise to make one church out of the nations will not fail in spite of our many differences and disagreements we see today.
Audio
ff to 4:45 for additional content