‘And if you are Christ's then you are Abraham's seed, heirs according to the promise.'
The idea of being ‘clothed with Christ' and indwelt by Christ is now connected back with the promise given to Abraham (Galatians 3:7-8; Galatians 3:16) and connected forward with the reception of the promised Holy Spirit (Galatians 4:6-7). Once a person is ‘in Christ', and Christ is in him, he is a child of Abraham and inherits the promise of blessing. He becomes the heir of Abraham's blessing. Christians become ‘the seed of Abraham' towards whom all the promises were made. They become the true Israel.
And it is Christians and they alone who do so. The corollary is that those who are not in Christ are not children of Abraham. Thus do we see the significance of the single seed (Galatians 3:16). In the final analysis the seed of Abraham comprises Christ and all those redeemed in Christ, whether of the Old Testament faithful or of the New. The church is the new Israel, the ‘Israel of God' (Galatians 6:16; Romans 2:28-29; Romans 11:17; Ephesians 2:19-20), replacing the old who had disqualified themselves by refusing to enter into the blessing (although they are still welcome if they will return through Christ).