Rom 11:28-29 As far as the gospel is concerned, they are enemies for your sake;
but as far as election is concerned, they are loved on account of the patriarchs, 29for God’s gifts and
his call are irrevocable.
How does later generations of Israelites being saved in accordance with God's promise to the patriarchal fathers prove that once a person is saved they are always saved?
2 Tim 1:9 He who has given us life and
has called us with a holy calling, not according to our works, but according to his will and the grace which has been given to us in Yeshua The Messiah before the time of the world,
How does this exclude the necessity for believing in order to be saved?
It is God's will that a person believe, and continue to believe, in order to be saved. That's how a person is saved according to God's will (1 Corinthians 15:1-2 NASB). God's will in election does not exclude the necessity for faith, and a continuing faith, in order to be saved.
John 6:39 And this is the will of him who sent me,
that I shall lose none of all those he has given me, but raise them up at the last day.
It's also God's will that ALL men be saved (1 Timothy 2:3-4 NASB, 2 Peter 3:9 NASB). But we know from that alone that God's will is not always done just because it is his will that it be done.
John 17:12 While I was with them,
I protected them and kept them safe by that name you gave me. None has been lost except the one doomed to destruction so that Scripture would be fulfilled.
Let's see if John meant that no one who is in Christ can ever be lost. He speaks more about God's protection in salvation here:
"9No one who is born of God practices sin, because His seed abides in him; and he cannot sin, because he is born of God. 10By this the children of God and the children of the devil are obvious: anyone who does not practice righteousness is not of God, nor the one who does not love his brother." (1 John 3:9-10 NASB bold mine)
"24As for you, let that abide in you which you heard from the beginning. If what you heard from the beginning abides in you, you also will abide in the Son and in the Father." (1 John 2:24 NASB bold and underline mine)
And we see that it is incumbent on the believer to not let that which he heard in the beginning be forgotten in order for God to protect him. So we see that God's protection, so that you won't be lost,
is conditional on you continuing to abide in that which you heard from the beginning.
You must continue to believe the gospel to the very end to remain in Christ and the Father in order to be saved.
That's the condition for salvation. So we know John did not mean in John 17:12 that a believer can never ever be lost by Jesus because Jesus keeps them apart from any requirements on the believer's part. The requirement is that you stay in Christ through a continued believing.
Rom 6:3 Or don't you know that all of us who were baptized into Christ Jesus
were baptized into his death?
Not sure what significance this may have to an OSAS argument.
But if you are suggesting that salvation is a done deal, completed in the past when we first believed and died in Christ, then I will say no one to my knowledge on either side of the OSAS debate says we are not born again when we first believe. The debate is whether or not a person will always be born again once they believe. The above passage by itself does not address that.