A few years ago, I had a friend named J B whom I think is typical of many people in our society, Even though J B was skeptical about the church and traditional Christian beliefs, he always expressed profound admiration for Jesus and specifically for the teachings of Jesus. He even had a favorite Bible verse that he would quote to me on occasion. However, J B didn’t believe that Jesus was God. He viewed Jesus merely as a great moral teacher—and perhaps the greatest moral teacher—but not as God in the flesh. And I think that is probably the most common perception of Jesus in our society today.

However, Jesus himself claimed to be God. We can see that over and over again just in the Gospel of John, not to mention the other books of the Bible that record Jesus’ ministry. In John 5:18, the Jewish religious leaders (who were trained in theology) conclude that Jesus was making himself “equal with God.” A few chapters later, in John 10:33, the Jews are again ready to execute him because “you, a mere man, make yourself God.” In John 20:29, when Thomas comes out of a period of doubt and refers to Jesus as “my Lord and my God,” Jesus actually mildly rebukes him for not coming to that conclusion earlier. And that only scratches the surface.

That’s why I love what C. S. Lewis writes with regard to this issue in his book Mere Christianity. He writes, “People often say, ‘I’m ready to accept Jesus as a great moral teacher, but I don’t accept His claim to be God.’ That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic…or else He would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God, or else a mad-man or something worse. You can shut Him up for a fool, you can spit at Him and kill Him as a demon, or you can fall at His feet and call Him Lord and God. But let us not come with any patronizing nonsense about His being a great human teacher. He has not left that open to us. He did not intend to.”

So, Lewis says, Jesus is either a lunatic who genuinely thought he was God and so made that claim with all sincerity, or he was a liar who knew full well that he wasn’t God but made that claim anyway, or he really was and is God. Those are our only three options. He can’t be merely a great moral teacher.

Now at this point, you may be wondering, “Why make such a big deal about this? Why does this have to be such an important issue?” Well, think about it. The main message of Christianity is that Jesus died on the cross to pay of our sins. We were sinful—we had offended a holy God by rebelling against him and were therefore entirely deserving of eternal punishment in hell—but Jesus died on the cross in order to suffer that punishment in our place. That’s the gospel. That’s the main thing Christianity teaches.

But here’s the thing: if Jesus wasn’t really God, how could his blood have sufficient value to pay for our sins? Let’s say he was just a moral person. How could the blood of a mere human—even an incredibly moral human—pay for the sins of the rest of humanity?

You know, if I write a check in order to pay for something, that check will only pay for the thing I’m purchasing if there’s actually money in my checking account. And if there’s not money in my checking account, that’s not good. That’s called writing a bad check, and if I write enough bad checks, I can actually get in a lot of trouble for that, because bad checks don’t have any value. They don’t actually pay for the product.

In a similar way, if Jesus wasn’t truly God in the flesh, then the blood he shed on the cross would not be sufficient to pay for our sins. So this isn’t some peripheral issue that doesn’t really matter one way or the other. Two people cannot disagree about this issue and both be true Christians.