Paul's analysis of Abraham's "offspring(s)" in Galatians 3

Paul sometimes chose to push the envelope a little, as he does here, and in some cases the outcome is an unsupported argument created in the cause of spreading the gospel. In 'Paul's "Use" of Scripture: Why the Audience Matters', published in As It is Written (edited by Stanley and Porter), pages 135-6, Christopher D. Stanley is speaking of Galatians 3:16 when he says:

Paul was aware of the limited biblical knowledge of his intended audiences and crafted his arguments to suit their capabilities. Instead of expecting his audiences to remember and reflect on the original context of his biblical references, Paul seems to have made a serious effort to communicate to his audiences how he wished for his references to be understood ... All that was needed was for an audience to be able to follow Paul's developing argument (no easy feat!) and to accept his interpretations of scripture as valid.

On page 134, Stanley says one can point to several places in Paul's letters where an ancient reader who was capable of consulting the original context of Paul's references might have concluded that the text of Scripture offered more support for the views of Paul's opponents than for Paul.

Because Paul is referencing Genesis 22:18 in speaking of the fulfillment in specific:

In your seed all the nations of the earth shall be blessed, because you have obeyed My voice.

In other words, Christ is the ultimate fulfillment of the covenant made to Abraham, though there have been mini-fulfillments throughout history.

_{^{See some cross-references to that verse, including the one you intially cited - http://www.biblegateway.com/passage/?search=Gen12:3;18:18;Acts3:25;Gal3:8,16&version=NASB}}

Warren's answer is good I thought it might be worth expanding on it though.

The best explanation I have heard is that the seed was a certain lineage that was gradually narrowed as the revelation of the Old Testament unfolded. It could never be all Abrahams many seeds as some provided no blessing to the world in any sense, but that special lineage down to the Messiah could be the only means from which a world could be blessed through Abraham. By the time Messiah was actually manifested in the flesh Paul declares that the seed is actually Christ only, for this lineage means nothing but as it lead up to him and terminated in him. Furthermore as the whole church, past, present and future gets its life in Him, in this single seed, represents all true children of Abraham.

Without following the argument along these lines it seems like a very poor argument and dishonest play on words. So it is not absolutely based on the word, seed versus seeds, but what seed meant versus seeds, that is the narrowing lineage until it hit its final mark.

Evangelicals defend this analogy by arguing (1) that even in the original context, "Abraham's offspring" did not refer to all Abraham's physical descendants, (2) that "Christ" can be interpreted as a spiritual descendant, not merely a single physical descendant, and (3) that the context, particularly Galatians 3:28–29, demonstrates that Christians are joined together in Christ as descendants of Abraham.

First of all, we know that "Abraham's offspring" cannot refer to all the descendants of Abraham: John Calvin points out that in Genesis 21:12, the offspring is limited to Isaac, and then later again Esau is excluded. Thus, Paul is arguing here that "offspring" cannot refer to all Abraham's biological descendants, but only those who are called by God. Calvin writes (using seed as a synonym for offspring):

He merely shows that the word seed must denote one who was not only descended from Abraham according to the flesh, but had been likewise appointed for this purpose by the calling of God.

So how do we get from "those who are called" to a single "offspring"? Like Calvin, Jamieson, Fausset, and Brown argue that the single "offspring" is the unified body of Christ, with Christ himself at the head:

one … Christ—not in the exclusive sense, the man Christ Jesus, but "Christ" (Jesus is not added, which would limit the meaning), including His people who are part of Himself, the Second Adam, and Head of redeemed humanity. Ga 3:28, 29 prove this, "Ye are all ONE in Christ Jesus" (Jesus is added here as the person is indicated). "And if ye be Christ's, ye are Abraham's SEED, heirs according to the promise."

That is, the singular "offspring" refers to Christ, considered generally: the redeemer, as opposed to the biological descendant of Abraham. Abraham's offspring, thus, is Christ, and all those who are joined to him through redemption, both from Abraham's biological descendants, and from his non-biological descendants. As JFB states, Galatians 3:28–29, just a few verses later, makes this plain:

²⁸ There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus. ²⁹ And if you are Christ's, then you are Abraham's offspring, heirs according to promise.

The Greek word translated "offspring" in v. 29 is the same word that is used earlier. Thus, by virtue of being "one in Christ Jesus," all Christians are Abraham's "offspring," in the singular sense of offspring.

The New Bible Commentary (1970) summarizes:

The divine agreement referred, says Paul, not to the plurality of Abraham's natural descendants, but to his offspring, seen to be Christ as the head of a new race. The sense is not to successive generations of men, but fulfillment in one kind of offspring, which is summed up in Christ. This interpretation is not evident in the language of Gn. 12:7; 13:15; 17:7, 8, and 22:18, but neither is it excluded.