Are the verses that speak of Christ’s death in 33 AD plain speech? Are they as plain as this statement:
Acts 16:31 And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house.
2 Peter 3:9b ...but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.
Or perhaps as plain as this:
Mark 16:16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
All kinds of people read these 'plain' statements and draw their conclusions (just as you have done with 1 Corinthians 15:1-5). Acts 16:31's plain statement leads them to the conclusion that if you believe and accept Christ you're saved---a wrong conclusion.
2 Peter 3:9 leads others to the conclusion that God doesn't want anyone, anyone at all to perish, and He is not willing for anyone to perish, so, they say Christ died for the sins of all people in the whole world----another wrong conclusion.
And Mark 16:16 seems simple and plain enough doesn't it? Believe and be baptized and you're saved. Very clear. So people follow the seeming plain statement made there and---come to a wrong conclusion concerning salvation.
Their failure is that they did not bother to harmonize all of these seemingly plain statements of Scripture with the rest of the Bible. And as a result have fallen into grievous error. You and others are doing EXACTLY the same thing with the few verses that seem to teach Christ paid for sins at the cross in 33 AD. You are failing the test because you are not being careful to harmonize that conclusion with all else the Bible says about it (the list I gave would be a good place to start).