15:11a Whether, then, it was I or they, this is what we preach,

So whether it was I or the other apostles who preached to you (plur.), we (excl.) preached that same message.
-or-
So it does not matter if I preached to you, or the other apostles preached to you. We (excl.) all preached the ⌊same⌋ gospel,

15:11b and this is what you believed.

And that is ⌊the message⌋ that you (plur.) believed.
-or-
and this is ⌊the gospel⌋ in which you placed your trust.

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