JESUS AS THE SERVANT OF GOD and HUMANITY
Romans 15.8-12
Indeed, first coming of Christ had only one purpose to serve people of the world by teaching them the Gospel of GOD (Spiritual Commandments of the Law of Moses), "the Truth of GOD," and because of it Jesus Christ was the Servant of whole humanity.
"Confirm the promises" means to confirm all the promises of GOD and the prophecies of the prophets given to Abraham and to his descendants, promises that contain assurance to the descendants of Abraham that they will be the people of GOD forever and the GOD will be their GOD forever.
Promises of GOD also contain disciplinary promises and following consequences for obedience and disobedience to GOD, blessings and curses, resurrection from the dead, salvation, and Eternal Life for those who are godly and dedicated their lives to GOD obeying His Statutes and Commandments.
Christ reminded people of Israel of all what was promises to them, many promises already have been fulfilled, and some that still standing revealed the future of things to come for entire world to know.
It should be clear to everyone that it is not the Jews joining Christians in Christ, but it is the Christians joining Jews in Christ. GOD Father and Christ already have been GODS of Hebrews from the beginning of existence of Abraham, who's descendants GOD also adapted as His personal people.
Promises and prophecies of GOD also relate and to Gentiles because they are also the subjects of GOD'S will and His Laws. Despite the fact that Jesus Christ communicated very little with Gentiles during His mission, He on the other hand commanded, urged, and directed His Apostles and His other followers to Gentiles, to preach for them His Gospel and save as many souls as possible, they are His people too.
In this sense Jesus Christ truly presented Himself as the Servant not only of His Father GOD Whom He faithfully serve from the beginning, but He became the Servant of the whole humanity of the world in His first coming, and He fully accomplished His mission.
For this reason, Paul presented following verses from 9 to12, "I will give praise to You among the Gentiles," verse 9, (2Samuel 22.50) (Psalm 18.49).
"Rejoice of Gentiles with His people," verse 10, (Psalm 117.1).
"There shall be come the Root of Jesse," verse 12, to Rule over Gentiles and in Him Gentiles will have Hope. (Isaiah 11.1-5.10) (Revelation 5.5).
~ Jesus Christ as the Servant of the Lord: (Isaiah 42.1, 52.13, 53.11) (Zechariah 3.8) (Matthew 12.17-18) (Acts 3.13.26) (Romans 15.8)
As the Christ serve us and even died for us in order to save our souls for His Father and His Kingdom, we shall do the same. The first worshipers and servant of GOD were the Jews. From them Torah (5 Books of Moses) became known to the world, following by the Prophets the men of GOD who are Preached and Prophesied to the world about One True GOD of Israel and His will.
~ Out of Zion shall go forth Instruction, and the Word of the Lord from Jerusalem: (Isaiah 1.27, 2.3, 42.4, 49.6, 51.4, 59.20) (Deuteronomy 18.15.18-19) (Micah 4.1-4) (Matthew 12.18-21) (Luke 24.47).
Leviticus 25.55
55. "To Me the people of Israel are servants, they are My servants whom I brought out from the land of Egypt, I am the Lord your GOD.” (Deuteronomy 32.36.43).
Because Jews and Gentiles fail their GOD, GOD will change His approach to humans as whole and will write His Laws and His will on their hearts (souls-spirits) and they will be possessed by GOD'S Law and will be obedient to Him forever.
~ New heart. Law of GOD written on the heart: (2Chronicles 30.12) (Psalm 40.7-8) (Proverbs 7.3) (Jeremiah 24.7, 30.22, 31.33, 32.38.40) (Hebrews 10.16-17) (2Corinthians 3.3).
So, in this case, obligation and responsibility of every true Christian is to do the same, to serve GOD as GOD serve us. To spread the Truth (not false teachings of different denominations and their false doctrines), the Gospel of GOD to all corners of the world and faithfully and loyally serve Him to the rest of our lives, with full obedience to His Laws, Statutes, and Commandments, with Fear, with Hope, with Faith, and with Love for Him.
~ Nations will come to Zion and will worship the Lord the GOD of Israel: (Isaiah 14.1, 18.7, 19.19-23, 55.5, 60.3.6-7.9-13.16, 66.23) (Jeremiah 3.17) (Zechariah 2.11, 8.22, 14.16-19) (Revelation 11.6-13, 15.4).
I remember one "preacher" said that Israel lost its privilege due to their disbelieve in Christ and for many other failures before GOD. He claimed that now "Christians" replaced Jews, and now they (the Christians) are the servants and the people of GOD. Of course this man know very little and even less understand the GOD, His compassion, His mercy, and His Love.
The Holy Scripture numerous times revealed that GOD never will abandon His people Israel under any circumstances, and even these few statements of Jeremiah fully confirm the faithful and loving nature of GOD to His chosen people Israel: (Jeremiah 31.35-37, 33.19-22.25-26).