The grafted in olive branches, partaking of the same "root and fatness" (KJV) is a similar analogy to the Temple with the same foundation, whose chief cornerstone is Christ. Speaking to Gentiles in Ephesians 2,...
Now therefore ye are no more strangers and foreigners, but fellowcitizens with the saints, and of the household of God; And are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone; In whom all the building fitly framed together groweth unto an holy temple in the Lord: In whom ye also are builded together for an habitation of God through the Spirit. (Eph 2:19-22)
- addressing Gentiles about their place in the Church
- the unity of Gentile and Jew in the same "body": in Ephesians, it is a Temple and in Romans, it is an Olive Tree
- The humility of the Gentile toward the Jews for being added after the Jew: Eph 2:11-13 and Rom 11:17-21
- Having the same "base": In Ephesians, it is the foundation and in Romans, it is the root
- Emphasis on the singular: Foundation vs Root
- being added by Faith (Eph 2:8) and (Rom 11:20)
In the Analogy of the Temple:
And are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone; (Eph 2:20)
I think the root would be the something similar, the root of the Apostles and Prophets, Jesus Christ being the Chief Root.
Some additional comments:
- It is interesting to note that all the Apostles and Prophets where distinctly Jewish (Rom 3:2) including Jesus.
- Another question arises: what is the "fatness"?
Alternatively, the Interpretation of Abraham being the root could stem from the analogy of the Seed of Abraham
Even as Abraham believed God, and it was accounted to him for righteousness. Know ye therefore that they which are of faith, the same are the children of Abraham. And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed. So then they which be of faith are blessed with faithful Abraham.
That the blessing of Abraham might come on the Gentiles through Jesus Christ; that we might receive the promise of the Spirit through faith. Brethren, I speak after the manner of men; Though it be but a man's covenant, yet if it be confirmed, no man disannulleth, or addeth thereto. Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
For ye are all the children of God by faith in Christ Jesus. For as many of you as have been baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus. And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
This time the Analogy (or maybe allegory) is Family.
The same similarities are also included:
- Addressing Gentiles
- Including Gentiles in the same body: Family vs Olive Tree
- Sharing a common source: Father Abraham vs Root
- the Emphasis on the singular: Seed and Root
- Being added by Faith
In this sense, Abraham the Patriarch certainly is a candidate, but it does not square well with the Temple Analogy, although there too the Household of God is mentioned). I think it is the Gospel (Gal 3:8) preached to Abraham, the Word of God (the foundation of Apostles and Prophets), the singular Seed which is Christ, the actual Promise given to Abraham for He is the "Greater than Abraham".