I started playing guitar at age 5. Now I'm 60. I'm a good guitarist, even though down in Nashville there are 10,000 guitar players better than I am (seriously). I know how to play guitar. Even though I have a 2-year degree in music theory guitar-playing, for me, is theory morphed into experience.
Some people play air guitar. Others play guitar hero. Neither air guitar nor Guitar Hero are anything like actual guitar-playing. Someone who knows how to play guitar, and can do it well, is in a different world than guitar wanna-be's.
There's a difference between theoretical knowing and experiential knowing. The beginning of theoretical knowing came, arguably, with Descartes in the 17th century. Descartes's philosophy posited a metaphysical distance between the knowing subject and the "object" to be known. With him the quest for "object-ive" knowing begins. Prior to Descartes knowing was participatory; post-Cartesian epistemology struggle with epistemic distance. One now wants a kind of mathematical certainty when it comes to knowing anything. This expectation creates problems for anyone who wants to "know." "Knowing" becomes a theoretical knowing-about rather than a participative knowing-how.
Hebrew "knowing" is a knowing-how more than it is a knowing-about. Old Testament scholar David Hubbard writes: “In the OT knowledge is living in a close relationship with something or somebody, in such a relationship as to cause what may be called communion.” (341) This is important to understand if one wants to understand Jesus-culture. The question "Do you know God?" is, in this Hebrew context, equivalent to "Are you intimate with God?" This is different from the question "Do you know about God?"
In the Gospel of John the word "know" is used 80 times, and always means experiential knowledge, or knowledge-by-aquaintance. As when, for example, in John 3:11 Jesus says to Nicodemus, "I tell you the truth, we speak of what we know, and we testify to what we have seen…" Without the "seeing" the "knowing" would not be there. In John 4:22 Jesus speaks to the Samaritan woman at the well and says, "You Samaritans worship what you do not know; we worship what we do know, for salvation is from the Jews." Jesus is informing the woman that she does worship something but it’s not experientially real. By analogy, she might as well be playing "air guitar."
Someone who believes in Jesus but does not know Jesus is only playing "air-religion." The real thing is experiential, relational, a knowing-by-acquaintance, and non-theoretical (in the sense that one has a theory about Jesus without the communion with Jesus). How important is this? Just this: it's experience, not theory, that breeds conviction. For example, just last week someone shared with me that they had an encounter with God after which things have never been the same in terms of their being released from a life-long feeling of shame and inadequacy. I happened to be in the room when that deliverance happened. I have seen the results. I just look at this person now and think, they are free from the weight that formerly oppressed them. Last week they told me, "Now I know I could never doubt God any more. If that is the only thing God does for me in my life I will remain forever grateful."
In John 17:3 Jesus says, "Now this is eternal life: that they may know you, the only true God, and Jesus Christ, whom you have sent." This is not about some future knowing; it’s knowing now. It’s hands-on, intimate, personal knowledge. (See here philosopher of science Michael Polanyi's Personal Knowledge.) Experientially it feel irrefutable precisely because it is not some abstract-theoretical knowing-from-a-distance.