You have no support at all from Scripture that believing for a while means loss of salvation.
These are those who never believed.
What you keep dismissing is that when one believes, they are sealed with the Holy Spirit as a promise for the day of redemption (Eph 1:13,14, 4:30, 2 Cor 1:22, 5:5), they have eternal life (Jn 3:16, 6:40), which means they cannot spend any time in the second death.
This speaks of assurance, not the FACT that one who has believed is secure. Being secure and knowing you are secure are 2 separate things.
Ok, switching gears here. This is a different context. It's about the tribulation. Does not apply to mankind in general. It's very specific.
Do you understand what Paul is saying here about the conditional statement of v.23?
He is saying that God reconciled us who have believed in order to present us believers holy and blameless. But ONLY IF we believers continue in the faith, and are not moved away from the confidence of the gospel.
This has nothing at all to do with OSAS.
13 But the ones on the rock are those who, when they hear, receive the word with joy; and these have no root, who believe for a while and in time of temptation fall away. Luke 8:13
These believed for a while the fell away.
4 For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, 5 and have tasted the good word of God and the powers of the age to come, 6 if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.
if they fall away...to renew them again to repentance.
And again -
12 Beware, brethren, lest there be in any of you an evil heart of unbelief in departing from the living God;
13 but exhort one another daily, while it is called "Today," lest any of you be hardened through the deceitfulness of sin. 14 For we have become partakers of Christ if we hold the beginning of our confidence steadfast to the end,
Hebrews 3:12-14
we have become partakers of Christ if we hold the beginning of our confidence steadfast to the end,
Steadfast to the end.
Those who endure to the end will be saved. Matthew 24:13
Those who believe for a while, then no longer believe, but turn away... Have become unbelievers.
They have fallen away, and it's impossible for them to be renewed unto repentance, for they have put the Son of God to an open shame.
We are required to remain steadfast unto the end... For the end of our faith is the salvation of our soul.
JLB