Jamieson, Fausset and Brown on 1 Corinthians 15:3

3. I delivered unto you--A short creed, or summary of articles of faith, was probably even then existing; and a profession in accordance with it was required of candidates for baptism (Ac 8:37).

first of all--literally, "among the foremost points" (He 6:2). The atonement is, in Paul's view, of primary importance.

which I ... received--from Christ Himself by special revelation (compare 1Co 11:23).

died for our sins--that is, to atone FOR them; for taking away our sins (1Jo 3:5; compare Ga 1:4): "gave Himself for our sins" (Is 53:5; 2Co 5:15; Tit 2:14). The "for" here does not, as in some passages, imply vicarious substitution, but "in behalf of" (He 5:3; 1Pe 2:24). It does not, however, mean merely "on account of," which is expressed by a different Greek word (Ro 4:25), (though in English Version translated similarly, "for").

according to the scriptures--which "cannot be broken." Paul puts the testimony of Scripture above that of those who saw the Lord after His resurrection [Bengel]. So our Lord quotes Is 53:12, in Lu 22:37; compare Psa 22:15, &c.; Da 9:26.