Being credible is simply the perception of being trustworthy and believable. One of the key reasons is that the brand represents a level of quality that they know and trust. A key factor is being consistent.
People are more likely to trust a business or person who acts in a similar manner most of the time.
For example, if your publish a newsletter to your customers or potential clients, aim for similar quality in each issue. Also, be consistent in your style of writing. Consistency applies to every aspect of your business. Don't change your guarantee terms every couple of weeks. Ensure that all your products or services consistently meet the quality and value standards that you are promoting.