"They believe; that's what grafts them in. Many Jews did not, so they were 'out.' It is not determined by ancestry anymore--if it was even then."
The Old Covenant was based upon that which is physical - the physical bloodline, physical circumcision of the men, physical sacrifice of animals for remission of sin, etc - which is also literal. And literal is similar to what Paul says in II Corinthians 3: 6, "Who also hath made us able ministers of the new testament; not of the letter, but of the spirit: for the letter killeth, but the spirit giveth life."
Paul uses the Greek word γραμματος. translated as letter, also in Romans 2: 27 and in Romans 2: 29.
"And shall not uncircumcision which is by nature, if it fulfil the law, judge thee, who by the letter and circumcision dost tgransgress the law?" Romans 2: 27
"But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God." Romans 2: 29
γραμματος, or grámmatos, is from γράμμα, or grámma, which is Strong's number 1121, meaning. "from 1125, to write, figuratively, to describe, write, a writing,i.e, a letter, note epistle, book -bill, learning, letter, scripture, writing, written."
Paul's use of grámmatos in Romans 2: 27, Romans 2: 29 and in II Corinthians 3: 6 is subtle, almost in a metaphoric way. By "letter" Paul is referring to that which does not transform the reader from the spiritual state of the natural man who does not receive the Holy Spirit to a regenerate spiritual state. The "letter" is understood by the unregenerate person, who does not grasp spiritual things very well.
Look at Hebrews 5: 12-14: "For every one that useth milk is unskilful in the word of righteousness: for he is a babe. 14. But strong meat belongeth to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil."
Those who are not weaned from "milk" understand the "letter" but not the "meat," which is harder to understand and not understood without the Holy Spirit.
And in this statement one of the founders of the theology called dispensationalism, Christian Zionism, separation theology, postponement theology dual covenant theology or the rapture cult, Lewis S. Chafer, admits that dispensationalism has "...changed the Bible from being a mass of more or less conflicting
writings into a classified and easily assimilated revelation of both
the earthly and heavenly purposes of God, which reach on into eternity
to come.." Lewis. S. Chafer, ‘Dispensationalism,’ Bibliotheca Sacra, 93 (October 1936), 410, 416, 446-447
Chafer, a founder of Christian Zionism, following John Darby and C.I. Scofield, claimed the Bible is a mass or more or less conflicting writings and that dispensationalism or Christian Zionism makes the Bible more easily classified and assimilated, or more easily understood. Christian Zionism makes scripture more easily understood by those who are in the spiritual condition of the natural man of I Corinthians 2: 14 - that which is of the "letter" and not of the "meat."