Going back to 1 Cor 15:3
3For I delivered unto you first of all that which I also received: how that Christ died for our sins according to the Scriptures,
This People that Christ died for in Paul's Gospel, is according to the scriptures, the scriptures during his time would have been simply the Old Testament.
So, what does scripture in the OT say about those Christ was going to died for ? Does the OT scriptures ever say that Christ was going to die for the sins of everyone in the world without exception ? No it does not, but always a particular People is in view.
By First OT analogy, lets take the Day of Atonement, for this was a Type of Christ's Death and for whom it would be for.
It was for God's Chosen People Israel, and that's it. Lets look at Lev 16:15-22
15Then shall he kill the goat of the sin offering that is for the people, and bring his blood within the veil, and do with that blood as he did with the blood of the bullock, and sprinkle it upon the mercy seat and before the mercy seat.
16And he shall make an atonement for the Holy Place, because of the uncleanness of the children of Israel and because of their transgressions in all their sins; and so shall he do for the tabernacle of the congregation that remaineth among them in the midst of their uncleanness.
17And there shall be no man in the tabernacle of the congregation when he goeth in to make an atonement in the Holy Place until he come out, and has made an atonement for himself and for his household and for all the congregation of Israel.
18And he shall go out unto the altar that is before the LORD and make an atonement for it, and shall take of the blood of the bullock and of the blood of the goat, and put it upon the horns of the altar round about.
19And he shall sprinkle of the blood upon it with his finger seven times, and cleanse it and hallow it from the uncleanness of the children of Israel.
20"And when he hath made an end of reconciling the Holy Place and the tabernacle of the congregation and the altar, he shall bring the live goat.
21And Aaron shall lay both his hands upon the head of the live goat, and confess over him all the iniquities of the children of Israel and all their transgressions in all their sins, putting them upon the head of the goat, and shall send him away by the hand of a fit man into the wilderness.
22And the goat shall bear upon him all their iniquities unto a land not inhabited; and he shall let go the goat in the wilderness.
We learn also from the Tabernacle that when the Hight Priest [ A type of Christ] would go into the Most Holy Place, that He wore a Breast plate Ex 28:21-29
21And the stones shall be with the names of the children of Israel, twelve according to their names, like the engravings of a signet; every one with his name shall they be according to the twelve tribes.
22And thou shalt make upon the breastplate chains at the ends, of wreathed work of pure gold.
23And thou shalt make upon the breastplate two rings of gold, and shalt put the two rings on the two ends of the breastplate.
24And thou shalt put the two wreathed chains of gold in the two rings which are on the ends of the breastplate.
25And the other two ends of the two wreathed chains thou shalt fasten in the two clasps, and put them on the shoulder pieces of the ephod at the front of it.
26And thou shalt make two rings of gold, and thou shalt put them upon the two ends of the breastplate in the border thereof, which is on the inner side of the ephod.
27And two other rings of gold thou shalt make, and shalt put them on the two sides of the ephod underneath, toward the forepart thereof, over against the other coupling thereof, above the embroidered girdle of the ephod.
28And they shall bind the breastplate by the rings thereof unto the rings of the ephod with a lace of blue, that it may be above the embroidered girdle of the ephod, and that the breastplate be not loosed from the ephod.
29And Aaron shall bear the names of the children of Israel on the breastplate of judgment upon his heart when he goeth in unto the holy place, for a memorial before the LORD continually. Ex 28:30 And thou shalt put in the breastplate of judgment the Urim and the Thummim, and they shall be upon Aaron's heart when he goeth in before the LORD; and Aaron shall bear the judgment of the children of Israel upon his heart before the LORD continually.
The writer of Hebrews states that God Preached the Gospel to Israel by means of these Typological Offerings and Offices of that People Heb 4:1-2
Let us therefore fear, lest, a promise being left to us of entering into His rest, any of you should seem to come short of it.
2 For unto us was the Gospel preached, as well as unto them; but the Word preached did not profit them, not being mixed with faith in those who heard it.
Yes, the very Gospel of 1 Cor 15:3-4 And so Our Sins is Limited to a particular Chosen People ! In fact, the Children of Israel are called the Church in the wilderness Acts 7:38
This is he, who in the church in the wilderness was with the angel who spoke to him on Mount Sinai, and with our fathers--the one who received the living oracles to give unto us,
Folks Christ died for the Church Eph 5:25, that is what Paul Preached !