Imagine a child born on the international space station. As much as gravity is a part of every second of our existence, it is completely alien to him. Every day he floats through the space he inhabits. Everything he lets go of continues in whatever direction it was going until acted on by some other force. Among other effects, his hip joints (our familiar ball and socket) have formed very slowly because of the lack of pressure from crawling and walking as an infant and toddler. He had to intentionally exercise with resistance bands from the time he could be taught or tricked into doing so. This child is now of an age where the beginnings of physics can be explained and, of course, one of the fundamental forces governing our universe is gravity. How would you explain it?
That part is actually pretty easy. One equation with a constant and 3 variables is all it takes. The idea is reasonably easy, but then how would you convince him it was real? He has no direct experience with the force. To be sure, you could give evidence. Orbits depend on gravity. Without an attractive force, all celestial objects would fly in straight lines rather than curve around a central body. Planets around the sun, the moon about the earth, even his own home continues around the earth he sees out the window everyday. The problem is that motion is hard to observe at such an enormous scale. What if he resists this idea? Couldn't all the objects we just mentioned be travelling along on parallel lines? Even if he's resistant he should be able to see the changes in the relative positions of the earth, the space station, and the moon. Parallel, straight paths won't work but circular paths are still not obvious. Who knows what lengths he might go to put the data together without gravity? School teachers could, no doubt, give testimony to the creative ability of stubborn children! The best and brightest of earth believed their home to be flat for a long time because you can't observe the curve while you're standing on it.
The most effective device would be a field trip. A few moments anywhere on the surface of the earth (to say nothing of landing!) should be more than enough to convince our recalcitrant student of the reality of gravity. Understanding would be unleashed as the experience broke down the barriers of resistance. A whole new world would be opened. Even if he suggested that this was a idiosyncrasy of earth, a few more trips should settle the debate. Experience on the moon and another planet (we're the ones doing the dreaming here, right?) would render resistance more amusing than substantive. Gravity is universal.
Too often, as Christians, we get stuck in the explanation and evidence stage when talking about our faith with an unbelieving friend or family member. Some of them accept and are transformed. Several others are heart-breakingly unconvinced. What's needed is experience. Here, of course, is the problem. Getting a child to experience gravity on the earth, moon, and Mars is simplicity itself compared to conjuring up an experience with God for someone else. We don't control such things. God has to encounter them by His own choice in His own way if anything we say about Him is true. The beauty is that, if what we say is true, this is exactly what God wants to do.
This is certainly not a reason to stop giving explanations and evidence. What if we took the child to earth before explaining gravity? He'd have no frame of reference to understand his experience. So it is with our unbelieving loved ones. We ought to give them the information we have so when God does show up, they understand what is happening. What I am arguing for is for a little peace. Your friends and family aren't dependent on you coming up with the perfect argument for them to come to faith. They are dependent on God drawing them to Himself. The pressure is not on you. There's enough courage involved in overcoming my anxiety to just have the conversation without adding to it the pressure of having to perfectly convince my friend through my own skill and passion. More importantly, too often I find myself poring over past conversations and imagining new illustrations and completely neglecting asking God to do His work. Faith is not easy. It's not easy to accept the first time and it's not easy to put into practice as we go along. God is not tame or safe, after all.