Here is a link to a paper by Dr. Mark Nanos. In this paper, Dr. Nanos re-integrates Paul back into his Jewish setting and brings out the prophetic perspective— mostly from Isaiah— that Paul used in his approach to bringing the Gentiles/Nations into fellowship with Israel under the rule of the One Creator God.
Reading Paul in a Jewish way
Following are a few highlights from the paper— I encourage you to read the paper itself. (It would help to have a Bible in hand and follow along reading the texts that Dr. Nanos references.)
... Paul wrote to his assemblies in a Jewish way. I do not mean simply to signify that Paul was from a Jewish ethnic and religious background, as commonly held. I mean that Paul continued to practice Judaism--and, moreover, that he continued to promote it to those who became followers of Jesus Christ, non-Jews as well as Jews.
I am among those convinced that these non-Jews were the target audiences of his [Paul's] letters. It is historically most probable that these non-Jews joined Jewish communities and learned to practice righteousness according to Jewish cultural norms that were derived from the interpretation of Torah over the centuries.
According to this view, Paul understood that Gentile believers were fulfilling prophecy written in the Hebrew Scriptures. If Gentiles joined with Israel as proselyte converts— no longer Gentile but now Jewish— then they would not be fulfilling the prophecies that described the nations joining alongside Israel and worshiping the one universal Creator God.
“Or is God the God of Jews only? Is he not the God of Gentiles also? Yes, of Gentiles also,”
(Romans 3:29 NRSV)
“And the LORD will become king over all the earth; on that day the LORD will be one and his name one.”
(Zechariah 14:9 NRSV)
Dr. Nanos sees Paul as promoting, in the book of Romans, an attitude of “living graciously towards those who are different from ourselves”. The Paul that he sees— a Jewish Paul practicing Judaism— is not trying to erase the boundaries between nations but is trying to teach them to live in harmony under the rulership of the God of Israel.
Christ following non-Jews are to celebrate the One God of all humankind within the midst of Jews celebrating this same One God. The non-Jews proclaim the mercy shown to them as former enemies of God by joining in joyful recitation of psalms together with Jews, who are doing so because of God's truthfulness to fulfill the promises made to them. The groups remain recognizably different, yet, in terms of expressing gratitude to God, they do so together in unison.