epistamai

In the New Testament this verb is, for all practical purposes, another way of saying "know." It sits very close to the ordinary Greek verb for knowing, οἶδα (εἴδω), and the two are used almost interchangeably. Its exact shade depends on what is known: a fact one is aware of, a statement one grasps, or a person or thing one is familiar with.

When the thing known is a claim or a matter, the word tilts toward "understand" or "comprehend," as when Peter says he neither knows nor understands, or when a false teacher "understands nothing." When the object is a person or a practice, it means "be acquainted with," as in the demon's "Paul I know" or Apollos "knowing only the baptism of John." It differs a little from γινώσκω (γινώσκω), the verb for knowing that grows out of experience; most often, though, it simply means being aware that something is so.

The word also carries a faint formality that a translation cannot easily show. Every time it appears in Acts it is on the lips of a speaker, never in the narrator's voice, and the writer seems to reserve it for the polished openings of speeches, the equivalent of "as you well know." It keeps a distant echo of its history too: long before the New Testament it was the Greek word for expert, scientific knowledge, the ancestor of our word "epistemology." That technical sense has faded, but writers can still turn it against opponents who claim mastery yet, in their words, understand nothing.