what about the 'if'
By which also ye are dsaved, if ye ||keep in memory †what I preached unto you,
I trusted the Lord believing all of the elements of the gospel of Christ; that Christ died for our sins, that He was buried and that He rose again the third day. Those who don't believe all of those, such as the resurrection are those who "believed in vain".
1 Corinthians 15:1 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
1 Corinthians 15:2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
1 Corinthians 15:3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
1 Corinthians 15:4 And that he was buried, and that he rose again the third day according to the scriptures:
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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.