‘Now to Abraham were the promises spoken, and to his seed. He does not say ‘and to seeds', as of many, but as of one, ‘And to your seed', which is Christ.'
Notice also, he says, that the covenant is made with Abraham and ‘his seed' (Genesis 12:7; Genesis 13:15; Genesis 24:7). Then he reminds them that the word for ‘seed' is a singular collective noun, and can mean one (e.g. Genesis 4:25) or many, but even when referring to the many it means the many seen as one. We can see from this, therefore, that God deliberately avoided a word that could be used in the plural and chose a collective noun (that is what Paul was saying by his seemingly inaccurate grammar). And that was because in this case, while God had a collective seed in mind, He also had a final single seed in mind. He had in mind the Messiah (Christ) as ‘the seed of Abraham' who would bring blessing to the world.
This argument is not quite as unreasonable as some have suggested. To Paul the whole human race could be summed up in one man (Romans 5:19). In the same way, he suggests, Abraham would also think of his descendants as one seed. Thus the ‘children of Israel' called themselves ‘Israel' because they were the seed of ‘Israel' (i.e. of Jacob - in theory if not in fact).
Actually, humanly (but not grammatically) speaking, there were ‘seeds' because Isaac alone became the bearer of the covenant seed (Genesis 17:19), while the seed of Ishmael, (while included later in a wider covenant), was excluded. So what a collective noun means is the summing up of everything in one example, and as we see this could be narrowed down. For regularly one man could represent the whole. Thus when David fought against Goliath as Israel's champion it was not just he who fought, it was as though the whole of Israel fought with him. And when Goliath was beaten the Philistines recognised that in their champion's defeat they too had been beaten, and they fled. The many were summed up in the one, and this was how the participants actually saw it. Isaiah uses this idea with regard to the Servant. The Servant was initially Abraham (Isaiah 41:1-8), and then the seed of Abraham (Isaiah 41:8 and regularly), and then because of the unfaithfulness of the whole the faithful of the seed of Abraham, his true seed (Isaiah 49:3), who was to ‘raise up the tribes of Jacob' (Isaiah 49:1-6), and finally it was the One Who was par excellence the seed of Abraham, the One Who gave Himself for the sins of His sheep (Isaiah 53). The one became the many, and then became the One, all incorporated in the one seed, the seed of Abraham.
So Paul saw Jesus Christ as the fulfilment of all that Israel was called to do. He was God's Champion, God's seed. The children of Israel were one seed, but they failed. However, from that one seed would come one man, God's Messiah, and through Him, and through those of the seed who would follow Him, would go God's blessing to the nations. In ancient thinking one man could represent a nation, and a nation could be represented in one man. That is what Paul is saying here.
And again in Daniel 7 Israel is like ‘a son of man' in comparison with the four wild beasts. It is ‘human' rather than being ‘beastly'. But this ‘son of man' also comes before God to receive his kingdom in a way which clearly represents one who stands for the many (Galatians 3:13-14). Their representative and king receives the kingdom (Daniel 7:13-14), and in him they receive it too (Daniel 7:27). This sums up Paul's argument as well. It is misrepresenting it to say that it is just playing with grammar.
Comparison may be made with the seed of the woman and the seed of the serpent in Genesis 3:15. The seed of the woman certainly represented mankind and the seed of the serpent the snakes that would be the curse of mankind. But behind the serpent lay a deadlier, mysterious foe. And he too, far more importantly, would be defeated by the seed of the woman, and we could add like Paul, ‘and that seed was Christ'. Indeed the defeat of that Serpent by Christ is one of the themes of the Book of Revelation.
So what Paul is signifying here is that it in the end it is to the one seed of Abraham that the promises were given, the One Seed Who represented the collective seed and in Whom they were summed up, just as the Servant is the One Man and is yet the many (Acts 13:47, for example, represents Christian preachers as the Servant). This included both past seed (Romans 3:25) and future seed. It is all summed up in Christ. For Paul certainly intends us to see that the promises of Abraham include the church of Christ as well as Christ Himself, although received through the One (compare Galatians 3:29 where he says so). That is the whole purpose of his argument.
Note that the whole argument also assumes that the church is now the seed of Abraham, the true ‘Israel of God'.