The various creeds of the Christian faiths appear to be confused with respect to the subject.  In some traditions, the text is “We believe”, in some it is “I believe”.  This does not seem to be a bone of significant contention (for once!), as very closely aligned traditions may differ in this small detail. It seems to make sense, though, as the Credo is professed together, in public, and the distinction between “I” and “We” is largely eradicated.