There is no question that all Christians are eternally secure. The root argument for this doctrine is ‘predestination’.
If one looks at individual Bible verses on other subjects, pulls them out of context and then tries to conclude eternal security ‘yes’, or ‘no’, one never really finds any certainty. On the other hand if one just looks at the doctrine which directly speaks to the subject all confusion vanishes instantly. There is no doubt. It is as certain as the doctrine of God existing.
For example, in Romans 8:30 we see the full certainty of salvation from beginning to end. From calling to glorification in heaven:
And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified. (Rom 8:30, ESV)
Now predestination means God chose a person ‘for salvation’ (not salvation and then destruction). He did this ‘before the foundations of the world’:
even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love he predestined us (Eph 1:4–5, ESV)
The doctrine of predestination means specific ‘individuals’ are called and that God knows them by name, calling each one. In fact every Christian are called ‘the elect.’ ‘The elect’ is not some mysterious concept but a title for any Christian.
So there is some common sense to be had here. If God chose you before your were born and called you ‘elect’ this is a ‘title of blessing.’ If God knowing you before you were born and knowing everything you will do in your life decided to given you a title that implies ‘great blessing’, God would be very much an evil liar if you ended up in hell, which is impossible. Therefore there is only one rational conclusion. All who are elect, that is Christians, are blessed, from the standpoint of eternity and nothing in time can ever possibly alter that.
Eternal security is pretty central to understanding the gospel. Those who reject the scriptures concerning the doctrine of election can still be among the elect but it can only cause much confusion in understanding the various scriptures that discuss the subject.