Okay, let's have a little closer look at the verses that you offered:
1Cor 15:1 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
4 And that he was buried, and that he rose again the third day according to the scriptures:
Notice the “if” word followed by a warning. Doesn't sound like an accomplished fact to me though it does address something of the method of salvation.
Those who don't believe in the three elements of the gospel declared and preached by Paul would be believing in vain. An example would be the "some" who did not believe in the resurrection of the dead:
1 Corinthians 15:12 Now if Christ be preached that he rose from the dead, how say some among you that there is no resurrection of the dead?
1 Corinthians 15:13 But if there be no resurrection of the dead, then is Christ not risen:
1 Corinthians 15:14 And if Christ be not risen, then is our preaching vain, and your faith is also vain.
1 Corinthians 15:15 Yea, and we are found false witnesses of God; because we have testified of God that he raised up Christ: whom he raised not up, if so be that the dead rise not.
1 Corinthians 15:16 For if the dead rise not, then is not Christ raised:
1 Corinthians 15:17 And if Christ be not raised, your faith is vain; ye are yet in your sins.
Now lets look at the next verse you offered:
1Cor 12:13 For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit.
This passage says nothing of our salvation nor the timing of it. On to the third passage you offered:
It's a positional truth for all who are saved!
Eph 1:13 In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,
14 Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.
Even when taken out of context, which this verse obviously is, there is still no direct indicator as to the timing of salvation but there is a very important clue. Once we believed, we were sealed by the spirit of promise which is a down payment of our inheritance to be received in full when we are redeemed. The question then becomes, when are we redeemed?
Again, a positional truth for those who have trusted the Lord after hearing and believing the word of truth, the gospel of our salvation. As to your question, we are already have redemption through His blood, the forgiveness of sins, according to the riches of his grace:
Ephesians 1:7 In whom we have redemption through his blood, the forgiveness of sins, according to the riches of his grace;
We (the saved) look for that blessed hope, and the glorious appearing of the great God and our Saviour Jesus Christ:
Titus 2:13 Looking for that blessed hope, and the glorious appearing of the great God and our Saviour Jesus Christ;
for the redemption of our body
Romans 8:22 For we know that the whole creation groaneth and travaileth in pain together until now.
Romans 8:23 And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body.
Romans 8:24 For we are saved by hope: but hope that is seen is not hope: for what a man seeth, why doth he yet hope for?
Romans 8:25 But if we hope for that we see not, then do we with patience wait for it.
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Galatians 1:3 Grace be to you and peace from God the Father, and from our Lord Jesus Christ,
Galatians 1:4 Who gave himself for our sins, that he might deliver us from this present evil world, according to the will of God and our Father:
Galatians 1:5 To whom be glory for ever and ever. Amen.
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Philippians 3:20 For our conversation is in heaven; from whence also we look for the Saviour, the Lord Jesus Christ:
Philippians 3:21 Who shall change our vile body, that it may be fashioned like unto his glorious body, according to the working whereby he is able even to subdue all things unto himself.
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1 Thessalonians 4:16 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
1 Thessalonians 4:17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
1 Thessalonians 4:18 Wherefore comfort one another with these words.
As to the timing of salvation:
Acts 16:30 And brought them out, and said, Sirs, what must I do to be saved?
Acts 16:31 And they said,
Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house.
Have you?