I have been deeply and profoundly drawn to how important and yet how neglected the doctrine of regeneration is to the church. I think a lot of the confusion over eternal security or the preservation of the saints has to do with not correctly understanding what happens in regeneration and this in turn skews our understanding of the church. Being born-again or regenerate is not something that we drift in and out of depending on the day. When one is born-again, you become a new creation in Christ:
Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come. 2 Corinthians 5:17
A new creation. Not a change in attitude but a change in nature, in substance. Where once there was a dead in sins and trespasses enemy of God with a heart of stone there is now after regeneration a new creature, a brand new creation just as the universe was once a new creation, a living, breathing child of God with a heart of flesh with the law of God written upon it. I don't want to get too deeply into this question because it is too complex for a quick note like this. However I was watching John Piper's take on 1 John 2:18-19 and I thought it was worth watching and passing on to others. As a reminder the verses in question say this:
Children, it is the last hour, and as you have heard that antichrist is coming, so now many antichrists have come. Therefore we know that it is the last hour. They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us. 1 John 2:18-19
As Piper points out, one can be part of the visible church and yet leave but that is not because their regeneration has been undone but rather because they were never one of us to begin with. Taken together with John's Gospel account I think John (the apostle, not Piper) paints a picture of the perseverance of the saints that answers the question of how we are held safe by Christ (being a new creation after being born-again, held in the omnipotent power of God, ex. John 10:28) as well as why it seems sometimes that people we thought were Christians turned out to not have been truly born-again in the first place. Here is the video, give it a watch because I think this sort of deep diving on a verse and drawing from it the doctrines that it portrays is very important to understanding the bigger picture...