Will,
Unless we are double minded, we profess what we believe. Taken seriously the credible profession is just that... it is a profession of belief and it is credible. Credible to whom? Mature spirit-filled believers who constitute the local church leadership usually.
What renders it credible? Well, we could go to lots of places but 1st John provides signs of conversion. In a local church setting this is a much more reliable system than might appear. It is amenable to investigation and to assessment and it is not normally "rushed". Frequently a candidate will instigate the request for baptism when his own understanding is led to it. With Christian charity and prayer it is not as unreliable as some might think.

Regards,
Dan