The believer is indwelt by God the Holy Spirit by His Volition, also known as His Soverignty.
He indwells us to complete a good work.
God does nothing that is good for nothingness. This means He would not continue to indwell us if there isn’t room for further sanctification of the believer.
Every believer, though, does fall out of fellowship. This happens whenever we sin. When we sin, our soul laments and falls back to guilt and sees that if the angels were condemned for falling away, surely we will face a fiery indignation. This simply acknowledges how we feel when we fall out of fellowship. It doesn’t mean we lose our salvation, not that we cannot be saved unless we never sin.
There isn’t a second sacrifice, but there is a rebirth which is when God creates our human spirit and places it in us at salvation, when we exercise a smidgeon more faith alone in Him alone than absolutely no faith whatsoever.
There might indeed be those who were never saved, who continue to reject Christ, but those who were saved cannot lose their salvation by anything they do, because they didn’t receive it because of anything they had done.
Forgiveness for the unbeliever occurs at the moment we face God and confess our sins to Him through faith in what Christ performed at the Cross. Forgiveness for the believer who is out of fellowship, also occurs when by 1John 1:9, we return to face Him, change our thinking and confess our sins, (known and unknown) to Him, and He is sure and just to forgive us those sins.
Then the Holy Spirit is free to act again, sanctifying our soul, when we intake His Word in our thinking.
If a believer falls away, then never returns, it isn’t because he doesn’t have the opportunity. Instead, if the fallen believer continues in his sin, even after Divine discipline, he will finally come to a point, where logistically in God’s Plan, he might not have any more paths by which he can carry on in God’s Plan. At that point, he may be taken out of the way and then God the Holy Spirit would not indwell the dead body. That doesn’t mean he lost salvation.
Please note, it is God the Father who elects, not the Son. Even the disciples chosen by the Son weren’t all elect. Those who were were further glorified as in the case of Simon Barjona, who replied to Jesus Christ as to who He was, and when he replied He was the Christ, Jesus Christ exclaimed that he was blessed, because it wasn’t flesh or blood (body or soul) that had revealed this to him, but it was God the Father.
This is amplified in other passages where unbelievers are warned that once they taste the work of Holy Spirit and the voice of God the Father, should they reject the Son, there is no second opportunity, because there is only one sacrifice. This is associated with the doctrine of efficacious grace and the first call by God the Father, with the work of God the Holy Spirit.