In Acts 10:34, Luke wrote:
Galatians 3:16 (KJV) 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
Genesis 17:19 (KJV) And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him.
H2233ï¿½ï¿½ï¿½ zera zeh'-rah From H2232; seed; figuratively fruit, plant, sowing time, posterity: - X carnally, child, fruitful, seed (-time), sowing-time.
Posterity POSTER'ITY, n. [L. posteritas, from posterus, from post, after.] 1. Descendants; children, children's children, &c. indefinitely; the race that proceeds from a progenitor. The whole human race are the posterity of Adam.
2. In a general sense, succeeding generations; opposed to ancestors. To the unhappy that unjustly bleed, Heav'n gives posterity t' avenge the deed
The word "posterity" is singular, but it refers to millions of descendants of Jacob, of whom Christ is only one.