Though the emphasis here seems to be on discipleship, the phrase about not being fir for the kingdom of God may put it in a different light. The New Testament deals with a class of people that is often ignored today. These are the people who seem to believe but quickly and permanently return to their old ways. The Bible teaches that these people were probably never saved.
1 John 2:19 states, "They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us." This verse is very clear. There are some who identify the absence of true faith in their lives by their ability to go back to the world. 2 Peter 2:20-22 compares them to a sow that returns to the mire; or, as we might say, the pig returns to the pig pen. But notice, the pig never became a sheep--it was still a pig. It just acted like a sheep for a period of time.
This is not to say that all backsliders are false professors of the faith. True backsliding can be done only by those who have truly gone forward. In other words, a backslider is someone who truly knows God. Yet, God tells us that a child of His is always His child and will be treated as His child. This means that a true child of God who backslides will be chastened as a child. God will scourge every son He receives (Hebrews 12:6). He who does not receive chastisement is not a true son, but is illegitimate (Hebrews 12:8).
How can we tell whether someone is backslidden or if he is not a true believer? We cannot always tell. However, I see two indications of true belief in one who has gone away from the Lord. First, he does not get away with it. That is, God will chasten him in order to bring him back to Himself. Second, he retains a desire to get right with God in his heart. aul talked about this desire to do right even though he kept doing wrong in practice (Romans 7:15-25). This will be the experience of true believers who get away from God. They will never have the same enjoyment of sin that they once had. They will deal with greater guilt than before and will have greater internal struggles. The Spirit of the Lord is in them and they cannot go back to "business as usual" in the world.