1 Corinthians 15:11

Whether then it be. or they, so we preach, and so ye believed.

'I or they' -Paul, or the other Apostles.

'No matter which of these competent witnesses the Corinthians examine, no matter to which of these notable heralds they listen, they will always hear the identical testimony and proclamation.' (Lenski p. 644)

'Once more..he is pressing on them that their current behavior and theology are out of step with those of the other churches (cf. 1 Corinthians 1:2; 1 Corinthians 4:17; 1 Corinthians 7:17; 1 Corinthians 11:16; 1 Corinthians 14:33). Thus, "what they believed" through his preaching when he was among them is the same gospel preached presently by him and by all the apostles. On the matter of their denial of the resurrection, therefore, they are following neither Apollos, nor Cephas, nor Christ; they are simply going off on their own.. (Fee p. 736)

We should note that all the apostles preached the same message!

THE RESURRECTION OF CHRIST/THE GENERAL RESURRECTION:

'With this paragraph the reason for the present argument is stated for the first time.' (Fee p. 738)

Not everyone in Corinth was denying the resurrection. Neither was this. denial of the resurrection of Christ. Rather, some in Corinth were denying the general resurrection of all believers (and logically unbelievers also) (1 Corinthians 15:12-13). God, speaking through Paul, points out that this is an inconsistent position to hold. One cannot logically reject the resurrection of believers, and yet still believe in the resurrection of Christ.