The Jews believed that they had a natural right to the privileges of God's kingdom. They called themselves, therefore, "the children of the kingdom."
In the Parables of the Wheat and the Tares, the Mustard Seed, and the Leaven, Jesus was speaking to the physical nation, which the parables clearly reflect.
We should not expect brethren to be perfect; we all sin. God has not given His People the prerogative to judge another member as a tare.