There is a very specific point Paul promotes in 1 Corinthians 9 when comparing the persistence necessary for a Christian to receive their crown that lasts forever, whether a literal crown is given only to those with eternal life or eternal life is figuratively the crown. That point is not that Christians have to outperform each other as athletes do because "slots" for salvation are not as plentiful as the people who seek them. Not at all! Rather, Paul uses the extreme dedication of a certain kind of athlete, who practices intently and relentlessly in order to win a prize, as a valid analogy for the dedication of the Christian, who must remain steadfast to the point of not disqualifying themself from the race. The prize lasts forever, Paul says, so in some way it has to do with the eternal life that the righteous and repent are to taste, but the apostle is stark about how he does not want to forfeit his prize.
The above passage is one of several (such as 1 Corinthians 15:2, Galatians 6:8-9, 1 Timothy 4:16, and 2 Timothy 4:7) where Paul is not indirect about saying or very strongly implying, in one way or another, that Christians can lose their salvation, their means of receiving eternal life. It is neither logically impossible nor Biblically false for someone to live in dedication to God with all of what that actually entails and then blunder so as to become "disqualified", unless he or she repents. Actually, with how often Paul deals with this issue alongside Christ's own instances of addressing the issue (including in Matthew 6:15 and 10:33), it would be difficult to read through the New Testament and ignore applicable verses unless someone is trying to sidestep or misinterpret them. We must run in such a way as to actually get the prize, or as Paul similarly puts it in 2 Timothy 4:7, we must finish the race. Disqualification from salvation is dreadfully a part of New Testament theology.
Lack of ongoing investment is catastrophic for one's moral standing and relationship with God; ceasing to care about truth (not even for pure reason's sake) and maintaining apathy after casual ethical failures absolutely can squander salvation. While life, and in the ultimate sense eternal life in the bliss of New Jerusalem, is never truly beyond the reach of any wicked being that still lives and thus can repent, refusal to submit to the truth is not automatically overlooked by divine mercy. So many people seem to almost rejoice in the illogical and unbiblical, hence heretical, idea that they are "saved" regardless of their philosophical alignment and moral character that they do not even entertain the possibility that they have only assumed this idea is true because of its obvious personal convenience.
The salvation of Yahweh and Christ from sin and its just punishment of death with no restoration to life (Romans 6:23) for all of eternity is not secured by insincere or misguided thoughts or prayers of self-interest by those who make assumptions, including those which favor their immutable salvation no matter what they do, assumptions driven by cultural conditioning or subjective allure. Such uncommitted, delusional people could easily hear Christ's words after the resurrection: "I never knew you. Away from me, you evildoers!" (Matthew 7:23).
But more than some people unexpectedly finding themselves cut off from God and Christ through the second death (which really is death and not endless torture), people who genuinely were rational, righteous, and committed to God and his Son (they are not the same entities!) can falter in any of these qualities. With flagrant, unresolved faltering comes the risk of someone losing salvation and not caring enough to rectify their standing by continuing in unrepentance, whether neglectfully or with aggressive intention. They can be denied the "crown" of eternal life they so diligently pursued up to a point because they disqualified themself.
Still, even a relevant passage with a very dire warning about how past righteousness will not exempt someone from death for later sins in Ezekiel 33 points to a basis for hope for someone who is wicked, even if they were formerly perfect in their ethical conduct. Just as the righteous person who errs is now wicked (33:12a, 12c, 13, 18), and if their wickedness is severe enough, God might bring about their death, the inverse is affirmed. The wicked person who becomes righteous is now restored or perhaps for the first time on the course to life (33:12b, 14-16, 19), with the greatest scale of such principles being that of eternal life and eternal death. The optimistic side of this does not affect how anyone can step away from their righteousness or their salvation by unrepentant sin, excluding themselves from what would have otherwise been theirs.