The concept of knowledge is used to certify epistemic agents as good sources (on a certain point or subject matter) for an understood audience. Attributions of knowledge and denials of knowledge are used in a kind of epistemic gate keeping for (epistemic or practical) communities with which the attributor and interlocutors are associated. When combined with reflection on kinds of practical and epistemic communities, and their situated epistemic needs for gate keeping, this simple observation regarding the point and purpose of the concept of knowledge has rich implications. First, it gives one general reason to prefer contextualism over various forms of sensitive invariantism. Second, when gate keeping for a select community of experts or authorities, with an associated body of results on which folk generally might then draw (when gate keeping for a general source community ) the contextual demands approximate those with which insensitive invariantists would be comfortable.