It turns out that there are two ways (at least) to consider the kingdom of God. One is to understand that there are tares in the wheat field that are really tares and really not wheat. They will be pulled up at the harvest, set aside and burned. They seem to have no ontological wheatness in them.
But then there are the branches in the vine that do not bear fruit, are cut out and burned. They really were branches, really united to the vine, and really partook of ontological branchness.
Sometimes those who are sons of Abraham are really sons of Abraham in one way and really sons of the devil in another way. Sometimes one's uncircumcision is circumcision to God and one's circumcision is uncircumcision. You have to watch and listen closely. But not because God wants to make this harder to understand. He wants to make a point about hearts and the absolute necessity of the new birth.
"God is perfect, but He is no perfectionist. He likes to mess with our heads. I suggest we let Him" - Wilson, AC, p70