Credibility is whoever is believed , usually wins the case.One can have the best of court cases, but if the Judge does not believe your client’s testimony, as there is no witnesses credibility, usually results in a lost trial.
The BC Court of Appeal in Faryna v. Chorney,  2 D.L.R. 354 (B.C.C.A.), . stated at page 357:
The credibility of interested witness,particularly in cases of conflict of evidence, cannot be gauged solely by the test of whether the personal demeanour of the particular witness carried conviction of the truth.