Sometime back, I also said that if the Reformed had come back with:
Christ has died, Christ is risen, Christ will come again
that this might also be a reasonable summary. You are the first individual to post anything along those lines. Very good.
One must be very careful with identifying the scriptures to which Paul is referring. 1 Corinthians was written relatively early and most probably before any of the Gospels were written - certainly before Paul had seen them.
Therefore he is referring to his own letters as well as the OT. The Apostles did recognize Paul’s letters as Scripture fairly early on, but I don’t know the relative dates.
Well, no doubt Paul is referring there to the OT scriptures as the prophetic foundation of the Gospel, but I believe that the Gospel of Matthew began to circulate around 50 AD, while Paul's letters to the Corinthians were probably not written for six more years [56 AD].
Thus he more than likely did include Matthew's Gospel as part and parcel of "the scriptures" to which he is referring, especially since Paul, in none of his letters, ever felt the need to cover the same ground of historical facts about Jesus's ministry on earth that Matthew, Luke, and Mark did. Those writings were simply taken as "scripture" or "the Gospel truth", so to speak, by all who read them.