"They answered him, We be Abraham's seed, Jesus answered them, I know that ye are Abraham's seed; but ye seek to kill me, because my word hath no place in you. I speak that which I have seen with my Father: and ye do that which ye have seen with your father. Jesus saith unto them, If ye were Abraham's children, ye would do the works of Abraham. Ye do the deeds of your father. Why do ye not understand my speech? even because ye cannot hear my word. Ye are of your father the devil, and the lusts of your father ye will do . He that is of God heareth God's words: ye therefore hear them not, because ye are not of God." (Joh 8:33-47)
I've extracted some of the pertinent parts of this dialogue between Jesus and the Pharisees to illustrate a point. The Pharisees claimed to be Abraham's seed and Jesus agrees with them. But, he then says that they are not Abraham's children. It's confusing - until you realize that two different things are being spoken of. There is first a physical "seed" and then a spiritual "seed". To be child of God is to have the spiritual seed of God's word in you. God's "seed" is truth, while the "seed" that gives rise to the children of the devil is "lies." Thus, the birth in the flesh does not determine the spiritual nature of someone. Any claim that a physical bloodline determines the person's status is in direct conflict with what Jesus said. It's like this:
"Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God. That which is born of the flesh is flesh; and that which is born of the Spirit is spirit." (Joh 3:5-6)
What indicates the spiritual parent of someone is whether or not he can receive and understand God's word. Throughout His teaching, Jesus made use of "seed" as a metaphor in more than one way. Seed indicates the word one receives and also the spiritual nature that is born as a result of that word. There are numerous examples of this in the teaching of Jesus.
"But he that received seed into the good ground is he that heareth the word, and understandeth it; which also beareth fruit, and bringeth forth, some an hundredfold, some sixty, some thirty." (Mat 13:23)
"Then Jesus sent the multitude away, and went into the house: and his disciples came unto him, saying, Declare unto us the parable of the tares of the field. He answered and said unto them, He that soweth the good seed is the Son of man; The field is the world; the good seed are the children of the kingdom; but the tares are the children of the wicked one;" (Mat 13:36-38)
"Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field: Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof." (Mat 13:31-32)
In these associated parables you see the multiple use of "seed." Seed represents the word of God, the good soil those that understand it. In the second parable (tares and wheat) the seed is associated with the "children of the kingdom." There is an overlap that helps us understand that what Jesus is referring to is not a physical relationship, but rather, a spiritual condition that springs from hearing the word of God. The kingdom of heaven consists in the teaching and those who receive it and are thus born anew through the spirit within. As Jesus said elsewhere, the kingdom of heaven is within you.