This is a good question. I've struggled with it some.
I want to be careful not to sweep away all those that believe Jesus and the indwelling of the Holy Spirit is insufficient to keep them saved.
At one time I also believed one could lose their Salvation. Eventually, through the Holy Spirit enlightening the Scriptures, and through more mature brothers and sisters in Christ , I saw the destructive folly that horrible doctrine of losing salvation wrought. Not only in myself, but MANY others.
Was THIS sin it? Had I walked away from Church for too long?
Does my anger, lustful thoughts, lack of help towards others prove that last sin was the one that made me fall away?
Oooh does the Devil revel in a child of God twisted up in their thinking like that! Makes them completely ineffective in bringing others to Christ through The Gospel.
But once the truth was revealed that was all a lie, as completely counter-intuitive to human logic and thinking as it was, this revelation actually set me free to be the Christian God intended!
Sin was greatly reduced as MY effort, and constant looking and thinking about me, and my sin, went away, replaced by looking at HIM, and HIS effort.
My desire to help and love others was greatly enhanced. It's not appropriate to talk about all the works I've done since then, as God alone get's the Glory. But suffice to say, it's been night and day.
Church became a joy, and not a chore. I could go on and on, but you get the picture.
So, just like I'm sure there are some who claim eternal security but haven't truly been born again, there are some who seem to be confused about who exactly keeps them saved, that actually are, but haven't been enlightened to the fact they will ever remain a born again child of God.
That's why I engage in these threads. Some are confused, like I was, and some are wolves, looking to injure the flock.