I will leave the details for someone more motivated this evening to document.
Suffice it to say, only God knows who is saved. The once saved, always saved camp believe that once the Holy Spirit is dwelling inside a person, they cannot go back. they may stumble but they will not fall away.
The always saved camp believes that a person can for a variety of reasons insincerely accept Christ. It is such person that can fall away since they were never saved in the first place.
The Apostle Paul, who wrote much of the NT, inspired by the Holy Spirit understood that what God begins, He completes. The same grace and power of God that saves us; is the same grace and power that keeps us. Neither are by our own works. I cannot save myself, and I cannot keep myself, only God can; and He has promised that He will. Can God die spiritually? No! When I was born again, God supernatually changed me, he gave me a divine spiritual nature, defined as the new man. It is a divine nature empowered with perseverance that will take me through to the very end.
"Therefore, there is no condemnation for those who are in Christ."
Once you're in, you're in, you've made the final decision. A decision that is sealed with the blood of Christ, with the Holy Spirit, and secured by the Father Himself. There is no turning back for those who are TRULY IN. According to John, those who turn back, were never really genuine. They professed, but never really possessed.