Since the thread has been split, I would like to take this opportunity to update the OP:
1 Corinthians 15:11
Whether, then, it is I or they, this is what we preach, and this is what you believed.
To maintain consistency, Calvinists must assume that that which Paul and the other apostles preached was:
A) only intended for believers;
B) only intended for the elect;
C) a modification of the gospel outlined in vv. 3b-4.
A) cannot be true because Paul never guarded against it - on the contrary, it was his ambition:
Romans 15:20-21
It has always been my ambition to preach the gospel where Christ was not known, so that I would not be building on someone elses foundation. Rather, as it is written: Those who were not told about him will see, and those who have not heard will understand.
In Acts there are many examples of the apostles preaching to unbelievers. Here is just one:
Acts 8:9-13
Now for some time a man named Simon had practiced sorcery in the city and amazed all the people of Samaria. He boasted that he was someone great, and all the people, both high and low, gave him their attention and exclaimed, This man is rightly called the Great Power of God. They followed him because he had amazed them for a long time with his sorcery. But when they believed Philip as he proclaimed the good news of the kingdom of God and the name of Jesus Christ, they were baptized, both men and women. Simon himself believed and was baptized. And he followed Philip everywhere, astonished by the great signs and miracles he saw.
Paul allows for the possibility that the gospel he specifies in vv. 3b-4 might be heard by unbelievers in the Corinthian church:
1 Corinthians 15:1-2
Now, brothers and sisters, I want to remind you of the gospel I preached to you, which you received and on which you have taken your stand. By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise, you have believed in vain.
Paul also says in v.11, that 'this is what you believed.'
B) cannot be true because we do not know who the elect are.
Finally, C) cannot be true because Paul never even hints at such a modification.
Whilst Calvinism demands that the gospel outlined by Paul in vv. 3b-4 is not to be preached to unbelievers, Paul himself had no such concerns.