Charles Spurgeon Commentary 1 Corinthians 15:11

"Whether then [it be] I or they, so we preach, and so ye believed." — 1 Corinthians 15:11 (ASV)

Therefore whether it were I or they, so we preach, and so you believed.

There was not a doubt about that. All the apostles, all the early Christians preached the resurrection of Christ; and the Corinthians, when they became Christians, believed it.