Many have been taught that once they are born into the kingdom of God, they can't be 'unborn' no matter how they live, and they will always be God's children. Some of them also believe that it is through baptism that one is born again, and once they have had that, they are safe. There are many verses quoted to prove this position, but I would earnestly suggest that you also examine the many other verses that point to the contrary. In brief, let me say that the Bible teaches that God would never let go from His side anyone who has been born again, but also that there is a dangerous possibility of our walking away from God.
To be born again is not physical like our human birth. Our relationship with God as our Father is on a spiritual level unlike the physical relationship with our human father. This spiritual relationship is based on grace (unmerited favour) from God's side, which is received by us through our faith in Him (Eph.2:8,9). It will get broken if we give up our faith. Paul reminds the Gentiles that if the Jews got broken off as branches and they got grafted in, they could also get broken off from the tree, if they lost their faith (Ro.11:19,20).
We may lose our faith for several reasons. Perhaps it is due to some great disappointment with God because He didn't answer our prayers as we expected, or He didn't act according to what we thought 'we had faith for'. It could be because we aren't willing to give up some sinful pleasure at any cost. Or maybe we can't figure out something about God and we conclude He can't be real or true. We all may know people who have given up their faith, or we ourselves may have faced strong temptations to doubt God. We mustn't forget that the Devil is going to try His best to confuse us and turn us away from God (1Pet.5:8).
That is why there are so many portions of the New Testament that warn us to be careful, alert, vigilant and to endure till the end. If it wasn't possible to fall away from the faith after we have been born again, wouldn't all these verses become meaningless?
There is also a direct warning from Jesus that we should be 'overcomers' if we don't want to risk our names actually getting erased from the Book of Life (Re.3:5). He is not one to scare us with false threats!
Solomon went after other gods towards the end of his life and served them (1Ki.11:4-7). Perhaps he repented before he died, but if not, what would God do to him (De.8:19)? Demas, one of the workers with Paul 'loved this world' and left him (2Ti.4:10). Again, we don't know how his life ended. But what would happen if he didn't repent? We aren't attempting to judge such people, because we have no right or knowledge. But we can take a warning from looking at such possibilities.
Isn't there a chance that we don't fully accomplish what the Lord had planned for us because we got diverted? It's also possible for us to lose out. That's why we must make every effort to endure till the end.