"I believe that Jesus was a moral man and a great teacher, but I don't believe He was God. Why do you believe that?"
Many non-Christians have no problem believing that Jesus actually existed and that He must have been a very moral person and a great teacher (since He has had such an impact on the world even after 1900 years). But, as they correctly point out, being a very moral person and a great teacher does not prove that He is God.
However, when we read the accounts of Jesus' life in the Gospels (the books of Matthew, Mark, Luke, and John), we find an interesting thing. It turns out that we
simply claim that He was a moral person and a great teacher! Jesus did not leave this open for debate. He said over and over that He is God, and He claimed to be able to do things that only God can do (such as forgive sins and grant people eternal life):
"Then Jesus said to her,
"Your sins are forgiven."
The other guests began to say among themselves, "Who is this who even forgives sins?" Jesus said to the woman, "Your faith has saved you; go in peace."" (Luke 7:48-50)
We can see that Jesus claimed to be God and claimed to be able to forgive sins and grant people eternal life, things which only God can do. The Jewish leaders understood that He was claiming to be God, and therefore they tried repeatedly to kill Him for blasphemy against God:
"The woman said, "I know that
(called Christ) "is coming. When he comes, he will explain everything to us."
Then Jesus declared,
"I who speak to you am he.""
"Jesus said to them, "My Father is always at his work to this very day, and I, too, am working." For this reason the Jews tried all the harder to kill him; not only was he breaking the Sabbath, but
he was even calling God his own Father, making himself equal with God."
"[Jesus is speaking]
I give them eternal life,
and they shall never perish; no one can snatch them out of my hand. My Father, who has given them to me, is greater than all; no one can snatch them out of my Father's hand.
I and the Father are one."
"When Jesus saw their faith, he said to the paralytic, "Son,
your sins are forgiven."
Now some teachers of the law were sitting there, thinking to themselves, "Why does this fellow talk like that?
Who can forgive sins but God alone?""
Jesus also accepted honor and worship that rightfully belong only to God:
"[At Jesus' trial] Then the high priest stood up before them and asked Jesus, "Are you not going to answer? What is this testimony that these men are bringing against you?" But Jesus remained silent and gave no answer. Again the high priest asked him,
"Are you the Christ,
the Son of the Blessed One?"
"And you will see the Son of Man [Jesus] sitting at the right hand of the Mighty One and coming on the clouds of heaven." The high priest tore his clothes. "Why do we need any more witnesses?" he asked.
"You have heard the blasphemy.
What do you think?"
They all condemned him as worthy of death."
"Again the Jews picked up stones to stone him, but Jesus said to them, "I have shown you many great miracles from the Father. For which of these do you stone me?" "We are not stoning you for any of these," replied the Jews, "but for blasphemy, because you, a mere man,
claim to be God.""
"[Jesus is talking to the apostle Peter] "But what about you?" he asked. "Who do you say I am?" Simon Peter answered,
"You are the Christ,
the Son of the living God." Jesus replied, "Blessed are you, Simon son of Jonah, for this was not revealed to you by man, but by my Father in heaven."" (Matthew 16:15-17)
If Jesus were nothing more than a great moral man then He would have had to correct those who tried to worship Him as God, but instead He
"[After Jesus died and was resurrected] So the women hurried away from the tomb [of Jesus], afraid yet filled with joy, and ran to tell his disciples. Suddenly Jesus met them. "Greetings," he said. They came to him, clasped his feet and
Then Jesus said to them, "Do not be afraid. Go and tell my brothers to go to Galilee; there they will see me."" (Matthew 28:8-10)
"[After Jesus died and was resurrected] Then the eleven disciples went to Galilee, to the mountain where Jesus had told them to go. When they saw him,
they worshiped him;
but some doubted. Then Jesus came to them and said, "All authority in heaven and on earth has been given to me."" (Matthew 28:16-18)
"[After Jesus died and was resurrected] A week later his disciples were in the house again, and Thomas was with them. Though the doors were locked, Jesus came and stood among them and said, "Peace be with you!" Then he said to Thomas, "Put your finger here; see my hands. Reach out your hand and put it into my side. Stop doubting and believe." Thomas said to him,
"My Lord and my God!"
Then Jesus told him, "Because you have seen me, you have believed; blessed are those who have not seen and yet have believed." Jesus did many other miraculous signs in the presence of his disciples, which are not recorded in this book. But these are written that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name." (John 20:26-31)
When you read about Jesus in the Gospels (the books of Matthew, Mark, Luke, and John) you will see that He led a morally pure life and that He taught everyone else to do the same. Since we agree that He was a very moral person (as you indicated in your question) then it is very unlikely that He was deliberately lying or trying to pull off a hoax. In fact, He willingly suffered one of the most painful forms of execution (being nailed to a cross) because of His moral character and because He had to fulfill His mission here on earth. Any normal human would have admitted his hoax rather than allow himself to be tortured and brutally executed! On the other hand, maybe He was crazy and only
He was God. Again, the moral purity of His life and the sane, simple, and consistent messages that He taught show that He was not a raving madman. If He wasn't a liar or an insane lunatic then the only other possibility is that He really is God! Jesus did
leave us the option of being able to claim that He was simply a moral man and a great teacher!
For more on this, please see:
More Than a Carpenter
by Josh McDowell, chapter 2 ("Lord, Liar, or Lunatic?")
Who Is This Jesus? by Michael Green, chapter 6 ("What Did He Claim?")