First of all if salvation can be lost, it can never be stated that a person
has been saved, seeing as salvation is from a future event. That is, one would not be able to say that their fate has been finalized if in fact that fate can change. Yet that Bible does use such rhetoric.
Eph 2:8,9 For it is by grace you have been saved, through faith— and this not from yourselves, it is the gift of God— not by works, so that no one can boast.
Jesus speaks of salvation from condemnation (also known as "justification") being finalized upon coming to faith.
John 5:24 "I tell you the truth, whoever hears my word and believes him who sent me has eternal life and will not be condemned; he has crossed over from death to life."
Secondly is the issue brought up in many places like the Eph 2:9 above and Romans 4 which indicate that justification is not conditioned upon one's performance. If salvation wasn't gained based upon one's performance, then neither can it be lost by one's performance. Else it would be like the justification under the law - salvation by works.
Romans 8 also indicates that neither the future nor the individual themselves can change their fate, having been saved.
Rom 8:38,39 For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, neither height nor depth, nor anything else in all creation (which includes the individual), will be able to separate us from the love of God that is in Christ Jesus our Lord.
But of course typically salvation by works Christians will point out verses that show a correlation between a person's performance and their salvation status. And I point out, as I have in many such threads, that such verses are diagnostic in nature and are not speaking of conditions to be saved or to maintain one's salvation status. That is, while behavior identifies whether or not one has been born of God, behavior doesn't cause a person to be saved or unsaved.
1John 3:9,10 No one who is born of God will continue to sin, because God’s seed remains in him; he cannot go on sinning, because he has been born of God. This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not a child of God; nor is anyone who does not love his brother.
As for persevering in faith, that also is an identifying mark of those who REALLY belong. That is REAL Christians in contrast to those mere masquerading as "Christians", there is
1John 2:19
They went out from us, but they did not really belong to us.
For if they had belonged to us,
they would have remained with us;
but their going showed that none of them belonged to us.