I think we can all agree that Paul reminds the Corinthians of the gospel he initially preached to them when they were unbelievers.
Verse 3-4
For what I received I passed on to you as of first importance: that Christ died for our sins according to the Scriptures, that he was buried, that he was raised on the third day according to the Scriptures,
Now you don't remind someone of what was previously the case by referring to what is currently the case if what is currently the case was not the case previously. If you do, then you have not actually delivered a reminder.
If Paul is using what is currently (i.e. when he wrote the letter) the case - 'that Christ died for our sins' (i.e. 'our' is Paul's and believing Corinthian's sins) then it must be the case that this phrase sufficed in expressing the gospel when it was preached to them before. If not, then Paul has not actually reminded them.
Paul makes no attempt to clarify any possible misunderstanding when he says:
'
For what I received I passed on to you as of first importance: that Christ died for our sins'
You don't say, 'For what I received I passed on to you as of first importance', and then switch to what is true now (i.e. at the time of the letter's writing) unless it applied beforehand.
I do wonder what exactly a Calvinist preacher would say to a crowd of unbelievers in view of the doctrines they hold. Taking into consideration Paul's vocabulary regarding the gospel, would the preacher (who is a Calvinist) say:
Christ died for our sins folks. Now when I say 'our' sins, I don't necessarily mean all men's sins. No, Christ only died for believer's sins. So you must believe in the Lord Jesus Christ to benefit from his grace. Now, please, don't get me wrong...please also understand that I don't necessarily say that all of you can believe. To believe in Christ, it must be the case that God chose you before the foundation of the world. If he didn't choose you, you will never believe and you will suffer in hell eternally....because God did not choose you.
This is the good news folks.
Espousing TULIP forces the preacher to keep tripping over himself with qualifications. Without such qualifications, his preaching is disingenuous. And it is clear that he ends up delivering bad news which actually means he has not preached the gospel at all.
Would a Calvinist please give us an example of the gospel that they would preach to non-believers? Please use the various terms that Paul employs in 1 Corinthians 15 in his reminder of the gospel.