A.T. Robertson Commentary 1 Corinthians 15:11

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

So we preach, and so ye believed (ουτως κηρυσσομεν, κα ουτως επιστευσατε). This is what matters both for preacher and hearers. This is Paul's gospel. Their conduct in response to his message was on record.