The Church, the Body of Christ is the Seed of Abraham, for its seen when we consider a couple of verses in Hebrews to start. Heb 2:11
11 For both he that sanctifieth and they who are sanctified are all of one: for which cause he is not ashamed to call them brethren,
This no doubt refers to Christ as the " He that sanctifieth" and the " we who are sanctified" are His Body the Church, and they are of One or they are One. The Head and His Members are One 1 Cor 12:12
12 For as the body is one, and hath many members, and all the members of that one body, being many, are one body: so also is Christ.
Col 1:18
And he[Christ] is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.
Now lets look again in Heb 2:16-17
16 For verily he took not on him the nature of angels; but he took on him the seed of Abraham.
17 Wherefore in all things it behoved him to be made like unto his brethren, that he might be a merciful and faithful high priest in things pertaining to God, to make reconciliation for the sins of the people
So He became One when He took on the seed of Abraham in the flesh, He identified in the flesh with those He is the Head of, they also being the members of His Body.
Christ is the Seed of Abraham Gal 3:16
16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
And His Body the Church is Abrahams Seed Gal 3:29
29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.