1 Corinthians 15:11

Therefore. Here Paul closes the comparison between himself and the other apostles (vs. 9, 10), and concludes that since all valid Christian testimony derives its power from God, the identity and personality of the human witness is relatively unimportant.

So we preach. What a bold affirmation of the unity of apostolic witness! All the apostles gave the same testimony about the resurrection of Christ; hence it was immaterial which of them had brought the message to the Corinthians. This principle is of universal application, and may be usefully remembered in the modern church. The human agent is merely a mouthpiece used by the Holy Spirit to convey truth to men, and if success crowns his efforts, the credit belongs to God (cf. ch. 3:6).

So ye believed. Paul reminds his readers at Corinth of their original acceptance of his doctrine, which was that of all the apostles.