“And this is how we know that he lives in us: We know it by the Spirit he gave us.” (1 John3:24)
“This is how we know that we live in him and he in us: He has given us of his Spirit.” (1 John 4:13)

John tells us not once, but twice, that the proof we’re in God and He’s in us, is the Spirit He has given to us. He doesn’t stop to explain the details of this so we’re left to work it out on our own. The problem for many us is this: if the Spirit isn’t perceivable in any sort of sensory way —then how can it assure us? It seems like the story of the man who was always snapping his fingers. Someone asked him why and he said, “Well, it keeps the elephants away.” When told there weren’t any elephants around he replied, “See—It’s working!“
Of course, the problem isn’t with John. It’s certainly not with the Spirit. The problem is with us—specifically in the way we think about things. Our technological culture has us so locked into the scientific, empirical approach to knowing (“seeing is believing“), that we forget about the other avenues by which truth can be obtained.
Take for example the logic that science employs in processing the data it gathers. Such reasoning isn’t subject to scientific verification, yet it is accepted and employed as a means of gaining knowledge. Jurors use it to sift through all different kinds of evidence (both physical and non-physical), to determine its truthfulness. Nobody ever thinks to question the use of reasoning because it is a self-evident truth. (To disprove it, you would have to use it and therefore end up validating what you sought to refute).
John concludes his gospel with the familiar scene between Jesus and Thomas. Jesus points out that Thomas believed because he saw, but “blessed are those who have not seen and yet have believed,” (John 20:29). In saying this, He recognizes two substantially different ways of knowing. One is to personally see something, and the other is through accepting the testimony of others.
John then builds on this by saying he has recorded certain signs performed by Jesus so that those who read his gospel can believe (v. 30-31). Though “believe” is not always the equivalent of “know,” I think it is here (see also Luke’s language in 1:1-4). John is telling us that an unbiased examination of his gospel will lead people to the conclusion that Jesus is the Messiah.
How does all of this help us understand how God’s gift of the Spirit provides us with assurance?
Suppose you have some friends who own a cabin on the lake. They ask if you would like to use it for a couple of weeks this summer. You accept their gracious offer and begin to plan your getaway. Sometime later, your friends let you know that in addition to the cabin, there’s a boat you can use. Now let’s suppose you’ve never “seen” any of this. Does the fact that they’re letting you use their cabin say anything to you about your relationship with your friends? Of course it does. Now, does their offer of the boat reassure you in regard to that relationship? Absolutely.
Construct your own scenario, but I think this is how it is with our Father and the Spirit He gives. Our relationship with Him operates in the realm of testimony (not sight/science), but it is nonetheless as real as anything we can see. Jesus’ death on the cross clearly demonstrates our Father’s love and Him giving us the Spirit reassures us of our relationship. Though we know both of these things through His word rather than through our senses, it is no sense less inferior. It is only a difference in perception, not reality.
I’ll close this piece by reminding us that while Jesus is on record as saying that both seeing and accepting the testimony of others are valid ways to arrive at faith, only one is blessed by Him and that is believing as a result of hearing the word. I would suggest that perhaps this is because so much of what we do as disciples involves trusting God’s word as opposed to seeing something for ourselves. (Even those who were able to “see” something, saw only a small part of what they were asked to believe). Paul will say that our walk is characterized by faith in what God tells us through His word rather than through seeing things ourselves (2 Corinthians 5:7).
We will do well to remember this is a world that walks by sight.