Editor's Note: The following is an excerpt from Questions Christians Hope No One Will Ask (With Answers) by Mark Mittelberg (Tyndale House, 2010). 

This was it—the day I was finally going to pop the question. 

After years of friendship and many hours hanging out together, I knew my feelings for Heidi had grown beyond merely "being in like"—the truth is, I was really in love with her! 

Was Heidi in love with me—enough to be willing to become my wife? That's what I was about to find out. I felt fairly confident, but as any guy in my shoes knows, until you actually hear her say "yes," you live with a certain amount of trepidation and doubt. 

When the moment came, I worked up the nerve and blurted out the question. Heidi's reply? After a brief hesitation—one that felt like a million years—she agreed to marry me! I don't want to imply that I was excited, but the fact that I shouted, "She said YES!" over and over probably gives away my true feelings. 

Was our love real? It certainly seemed to be on that day. As it did on the day of our wedding. And when each of our kids was born. And when Heidi brought me freshly brewed coffee this morning. After more than twenty-five years of marriage, I think we've made a pretty strong case: our love for each other is genuine. 

Love is not a physical entity, and yet it's very real. In fact, for those who are in love, it can be more real than the world around them! But in order to know if there is true love in a particular situation, sometimes we need evidence. And being the skeptic that I am, I needed fairly strong evidence. 

In my relationship with Heidi, evidence of her love emerged along the way—she wrote me notes that reflected her affection; she spent hours with me on the phone; she seemed to enjoy being around me; she even gave me loving looks sometimes. Then there was the big day when she agreed to marry me. While each one of these actions pointed to her love for me, taken together they provided overwhelming confirmation. I could put it like this: the cumulative evidence was more than enough to believe that Heidi's love for me was the real deal

But can I prove it to you? Can I show you our love for each other in a tangible way—one that you can see, hear, or touch? No, the love itself is invisible. It's one of those things that you have to detect through its effects. Much like air: You can't see it (unless you're in downtown Los Angeles), but you can breathe it, experience it, and move in it. Or like gravity—it's not visible, but you'd better not try to ignore it! 

The Invisible God 

One of the most important issues that surfaced in the survey we talked about in the introduction—in fact, tied for first place as the question respondents most hoped nobody would ask them—was this: how can you know there's a God? He's not tangible; you can't weigh him, measure him, touch him, or see him with the naked eye—or detect him with radar, for that matter! His presence doesn't register with any of our senses, and yet you believe in him. Why? 

It's a challenging question that's obviously central to all we believe as followers of Christ. So how can we respond? 

First, we can point out to our friend, as I did above, that there are plenty of important things we believe in without seeing, hearing, or touching them. Love, as I've explained, is a profound reality, and most of us believe in love. But love itself is not a material thing. It's not something we can see, hear, or touch directly. 

The Christian understanding is that God is not a material thing either. This is clear in John 4:24, in which Jesus tells us "God is Spirit." Unlike my friends, my dog Charlie, my iPod, or my mountain bike—all of which I can see, hear, and touch because they are physical, material things—God is a spiritual being or reality, and spiritual realities are not the kinds of things that can be seen with physical eyes or heard with physical ears or touched with physical hands. So I guess we shouldn't really be surprised that we can't experience God in the same way we can experience those other things.