What did Paul pass on 'as of first importance' in 1 Corinthians 15?

Answer: That Christ died for our sins

Of first importance: Christ died for our sins according to the Scriptures, was buried, and was raised on the third day.

“For what I received I passed on to you as of first importance: that Christ died for our sins according to the Scriptures, that He was buried, that He was raised on the third day according to the Scriptures,”— 1 Corinthians 15:3-4 (BSB)