Paul sums up the section before turning to the problem of disbelief in the resurrection of the dead.

They refers to the other apostles.

Whether then it was I or they may be expanded to “whether it was what I preached or what the other apostles preached.”

So may be expanded to “this is the message that.”

We: the context makes it clear that Paul is speaking of himself and other apostles or evangelists who had worked at Corinth. The Corinthians are excluded.

You believed, as in 1 Cor. 15.2, refers to the Corinthian Christians’ first decision to believe. This phrase may also be rendered as “this is the message that you believed.”

An alternative translation model for this verse is:
• Therefore, whether it was what I preached or what the other apostles preached, this is the message we continue to preach and which you believed.

Quoted with permission from Ellingworth, Paul and Hatton, Howard A. A Handbook on Paul’s First Letter to the Corinthians, 2nd edition. (UBS Handbook Series). New York: UBS, 1985/1994. For this and other handbooks for translators see here .