I see the term pillar as having to do with being a protector of Jerusalem and the Temple, like in Rev. 3:12:
The one who is victorious I will make a pillar in the temple of my God. Never again will they leave it. I will write on them the name of my God and the name of the city of my God, the new Jerusalem, which is coming down out of heaven from my God; and I will also write on them my new name.
This is the same word Paul uses to describe James and company in Gal. 2:9, as Dunn notes here:
https://books.google.com/books?id=A_Ngb ... em&f=false
And Hag. 12b applies the term to a righteous one (which is another term used to describe James) in an interpretation of Prov. 10:25:
And the Rabbis say: The earth stands on twelve pillars, as it is stated: “He set the borders of the nations according to the number of the children of Israel” (Deuteronomy 32:8). Just as the children of Israel, i.e., the sons of Jacob, are twelve in number, so does the world rest on twelve pillars. And some say: There are seven pillars, as it is stated: “She has hewn out her seven pillars” (Proverbs 9:1).
Rabbi Elazar ben Shammua says: The earth rests on one pillar and a righteous person is its name, as it is stated: “But a righteous person is the foundation of the world” (Proverbs 10:25).
https://www.sefaria.org/Chagigah.12b.3?lang=bi
And Hegesippus uses a related term to describe James in EH 2.23:
Because of his exceeding great justice he was called ... Oblias, which signifies in Greek, ‘Bulwark of the people’ ...
Regarding the meaning of Oblias, Plumtree suggests it is derived from Ophel, which is what I am leaning towards as well:
The name Oblias, with the explanation which Hegesippus gives for it, represents the reverence felt by the population of Jerusalem for one who was to them the last surviving representative of the saintly life, and which shewed itself in their feeling that when he was murdered their defence was gone ...
[Footnote 2 re: the origin of the term Oblias]: The probable Hebrew form of the word Ophli-am (= stronghold of the people), the first half of the word being identical with Ophel, the tower on the south side of the Temple, which was the residence of the Levites (Neh. XI.21).
https://books.google.com/books?id=WWJbA ... el&f=false
This idea is echoed by Pixner in
Paths of the Messiah and Sites of the Early Church from Galilee to Jerusalem:
https://books.google.com/books?id=bvhA6 ... as&f=false
The term Ophel is used five times in the OT and means "a fortified mound or hill in Jerusalem."
http://biblehub.com/hebrew/6077.htm
As Bauckham writes in
James The Just and Christian Origins:
... the term Oblias and its interpretation should be seen in the context of the early church's understanding of itself as the eschatological Temple. Like the Qumran community, which also understood itself as a Temple, the first Christians could describe themselves and their leaders as various parts of the structure of the Temple building. Christians in general were the stones of which the Temple is constructed (1 Pet 2:5; Hermas
Vis. 3;
Sim. 9); the apostles and Christian prophets were the foundation (Eph 2:20); Peter was the rock on which the Temple was built (Matt 16:8); Jesus Christ was the foundation (1 Cor 3:11) or cornerstone/keystone (Eph 2:20; 1 Pet 2:4, 6-7). That this kind of imagery goes back to the early Jerusalem church can be seen from the designation of James, Peter and John as pillars (Gal 2:9), i.e., supports on which the messianic Temple building rests. We should note that in many cases specific references to these architectural facets of the eschatological Temple were found in scripture ...
https://books.google.com/books?id=5SHbj ... as&f=false
And Eusebius writes regarding this idea of James and company being protectors of Jerusalem in EH 3.7.9:
But it may be proper to mention also those events which exhibited the graciousness of that all-good Providence which held back their destruction full forty years after their crime against Christ — during which time many of the apostles and disciples, and James himself the first bishop there, the one who is called the brother of the Lord, were still alive, and dwelling in Jerusalem itself, remained the surest bulwark of the place.
Also see Dennis re: the definition of pillars in
The Encyclopedia of Jewish Myth, Magic and Mysticism: Second Edition:
https://books.google.com/books?id=WlDzC ... ld&f=false
You know in spite of all you gained, you still have to stand out in the pouring rain.