The Sower of the "Seed"

Descendants were given to Abraham by other than natural means, so that through him Messiah might come, and the point of the apostle’s argument is that since the fulfillment of the promises of God is secured alone by Christ, they only who are “in Christ” can receive them; of spiritual offspring, Rom. 4:16, 18; 9:8; here “the children of the promise are reckoned for a seed” points, firstly, to Isaac’s birth as being not according to the ordinary course of nature but by divine promise, and, secondly, by analogy, to the fact that all believers are children of God by spiritual birth; Gal. 3:29.
As to 1 John 3:9, “his seed abideth in him,” it is possible to understand this as meaning that children of God (His “seed”) abide in Him, and do not go on doing (practicing) sin (the verb “to commit” does not represent the original in this passage). Alternatively, the “seed” signifies the principle of spiritual life as imparted to the believer, which abides in him without possibility of removal or extinction; the child of God remains eternally related to Christ, he who lives in sin has never become so related, he has not the principle of life in him. This meaning suits the context and the general tenor of the Epistle.

Vine, W. E., Unger, M. F., & White, W. (1996). Vine's complete expository dictionary of Old and New Testament words (Vol. 2, Page 557-558). Nashville: T. Nelson.