He contrasts the battle between those possessed by the Spirit and those who are earthbound and tied to the flesh. He insists that only the Spirit could give true life. Jesus knew that the followers had seen the wonders and healings, which were signs of the kingdom. He also knew that they did not truly understand these signs nor did they really believe in him. Jesus, therefore, insists that only those who have been truly called by God can come to faith.
Faith is never easy.
It tests us. Therefore, some accepted him and others did not. John then quietly tells us that some, really many, of his followers left him and returned to their old life. We now hear Jesus directly question the twelve. They are the specific individuals we hear, in the Gospels, called by name.