Whether I or they Having compared himself with the other Apostles, he now associates himself with them, and them with him, in agreement as to their preaching. "I do not now speak of myself, but we have all taught so with one mouth, and still continue to teach so." For the verb kerussomen (we preach) is in the present tense -- intimating a continued act, or perseverance in teaching. [1] "If, then, it is otherwise, our apostleship is void: nay more -- so ye believed: your religion, therefore, goes for nothing."

Footnotes

1 - "Perseuerance a enseigner ceste mesme chose;" -- "Perseverance in teaching this same thing."

Therefore, whether it were I or they - I or the other apostles. It is comparatively immaterial by whom it was done. The establishment of the truth is the great matter; and the question by whom it is done is one of secondary importance.
So we preach - So we all preach. We all defend the same great doctrines; we all insist on the fact that the Lord Jesus died and rose; and this doctrine you all have believed. This doctrine is confirmed by all who preach; and this enters into the faith of all who believe. The design of Paul is to affirm that the doctrines which he here refers to were great, undeniable, and fundamental doctrines of Christianity; that they were proclaimed by "all" the ministers of the gospel, and believed by all Christians. They were, therefore, immensely important to all; and they must enter essentially into the hopes of all.

Whether it were I or they - All the apostles of Christ agree in the same doctrines; we all preach one and the same thing; and, as we preached, so ye believed; having received from us the true apostolical faith, that Jesus died for our sins, and rose again for our justification; and that his resurrection is the pledge and proof of ours. Whoever teaches contrary to this does not preach the true apostolic doctrine.
Paul was the last of the primitive apostles. The primitive apostles were those who had seen Christ, and got their call to the apostolate immediately from himself. There were many apostles after this time, but they were all secondary; they had a Divine call, but it was internal, and never accompanied by any vision or external demonstration of that Christ who had been manifested in the flesh.

Therefore whether it were I or they,.... By whom Christ was seen first or last, we were all eyewitnesses of him; or whether I am the least, and others the chief of the apostles; or whether I have laboured more abundantly than they all, this matters not:
so we preach; we agree in our ministry to preach Christ, and him only, and with one heart and mouth assert, that he died, was buried, and rose again the third day:
and so ye believed; these several truths relating to the death, burial, and resurrection of Christ. Thus the apostle, after he had made a digression upon his own character, as one of the witnesses of Christ's resurrection, returns to the subject he set out upon in the beginning of the chapter, in order to lead on to the doctrine of the resurrection of the dead, which he proves by the resurrection of Christ, in the following verses. One of Stephen's copies read, "so we believed"; and so the Ethiopic version seems to have read; see 2-Corinthians 4:13.

whether it were I or they--(the apostles) who "labored more abundantly" (1-Corinthians 15:10) in preaching, such was the substance of our preaching, namely, the truths stated in 1-Corinthians 15:3-4.

Whether I or they, so we preach - All of us speak the same thing.