This "failed to regenerate" argument is nonsense and a convenient talking point to avoid the real issues. When a man fails in his Christian walk, we simply say it was obvious that he was never a Christian to begin with... hogwash. God never made us into robots... we still have our free will. That free will also gives us the ability to reject our salvation. Don't forget that Lucifer made such a choice after being the brightest among the angels... and Adam made that choice to precipitate the Fall. Before those horrible choices, these beings were with God.
Before you say Hogwash - go back and read the parable of the soils again. Jesus categorizes different types of people. Some will hear the word and accept it but fall away quickly, some will accept it but will be pulled away by worries of this world, and some will accept it and will perservere in the faith.
Jesus commanded us to be born again or as some translate it to be born from above. That is a supernatural occurance where a person becomes a new creature as the Bible describes it. Jesus also said in the parable of the Good Shepherd that those that are truly His hear His voice and follow Him and no one can snatch them out of His hand.
There are true believers that have been born again who will never truly fall. They may stumble from time to time as all humans do, but a person who has a spritual new birth is indwelt by the Holy Spirit and will never lose their faith.