1) What are the plural promises?
Paul refers later to the singular promise of Abraham (Gal 3:17). Why does he refer to the plural promises here? Either he refers to each aspect of the promise to Abraham as individual promises (i.e., land, descendants, and blessing), or, more likely, he refers to promises because the promise is given multiple times. Repeatedly promised were descendants, blessing, and land (Gen 12:1–3, 7; 13:15–16; 15:5, 18; 17:8; 18:18; 22:17–18; 24:7; 26:3–4, 24; 28:4, 13–14; 32:12; 35:12; 48:4, 19). Also promised was the defeat of enemies who stood in the way of seeing those promises fully realized (Gen 22:17b; 24:60).
2) Where do we find “And to your offspring” in the OT?
If we demand that every word is an exact quotation from the OT and specifically the “and” in this phrase, we likely have to choose between Gen 13:15 or 17:8 as the source of our quotation. And, since Paul is speaking about the Abrahamic covenant, this issue is more at the fore in Gen 17:8.
At the same time, Paul has previously conflated and mixed quotations earlier in Galatians 3 (see Gal 3:8 with Gen 12:3 and 18:18; also see Gal 3:10 with Deut 27:26 and 28:58). So, if we allow some flexibility with this quotation here and do not demand that it must be exact in its correspondence to its OT source (particularly the “and”), possible sources could be Gen 12:7, 15:18, 22:18, or 24:7. Gen 22:18 is a favorite for some because of its correspondence to the context of Galatians 3—both passages speak of universal blessing that comes through the offspring of Abraham.
However, one takes it, Gen 17:8 seems to be the best source for the quotation, but it is a representative quote that assumes more is promised than just the land in that one verse. The quote brings to mind all the promises and everything that was promised, which is important to our next question.
3) How does Paul get Christ out of a promise made to Abraham?
Wherever we source our quotation in answering the question above, the issue here is what Abraham would have understood in the word offspring when he heard God’s promise spoken to him (and Moses, too, for that matter, since he recorded the matter). Paul identifies the offspring as Christ. Did Abraham also think that Christ was the recipient of the promise that worldwide blessing would come to all through Him?
Perhaps the best way to understand the promise to this offspring is to understand offspring in a generic sense and yet realize that it also referred to one specific male offspring who would see the promise realized in full. This understanding seems to be how Abraham and others would have understood it then, which explains why Paul would have identified the offspring as Christ as well.
The recipient of Abraham’s promise was his offspring, which could refer to anyone who was an offspring of Abraham. Whether to Isaac, Jacob, or Israel collectively as a nation, they, too, received the promises first given to Abraham. And, the blessing to the nations through them would have come by these nations blessing them and having faith in the promise themselves (Gen 12:3). The Offspring who received the promise to Abraham and brings blessing to the nations in its fullest form is obviously Christ because of what He did on the cross. By blessing Him, so to speak, through believing in Him, anyone among the nations of the earth may find himself blessed in that he is declared righteous before God through his faith.
The term offspring in two other instances is used similarly. First, Gen 3:15 promised that the seed of the woman would strive with but eventually crush the serpent. While it was singularly “he” who would crush the serpent’s head (obviously Christ), we find later that Satan’s crushing also involves the feet of believers as well (Rom 16:20). The struggle between Christ and Satan is shown through the plural offspring of each throughout the ages, as seen as early as the struggle between Abel and Cain in Gen 4 (cf. 1 John 3:9–10, 12). So, while Christ is the singular Offspring who crushes Satan in the end, so also all the offspring of the woman who are in Him will enjoy victory over Satan as well. Offspring can refer to both Christ and many offspring.
Second, 2 Sam 7:12–13 promised to David that his offspring would have his kingdom established and that the throne would be established forever. While some of David’s offspring were kings who saw their thrones established in part, the greatest Offspring of David is obviously Christ who will come again and rule forever. Again, offspring here refers ultimately to Christ and other offspring before Him.
Added to all of the above is the fact that we have seen at least three stands of promises to offspring that all find their greatest fulfillment in Christ. The original readers of the OT would easily have understood the offspring from one promise to the other to find partial fulfillment in their day, knowing that one day the promise’s greatest fulfillment would be in the greater Offspring to come.