If Jesus knew he wasn't God:
How could a liar be a great moral teacher?
However, when you read about what Jesus did and said, he doesn't come across as a liar.
Jesus said and did some wonderful moral things. For example, Jesus commanded his followers to love their enemies, not to judge others and He even stepped in when an adulteress was about to be stoned.
Also, Jesus did some very miraculous things. For example, from Sunday school, many would remember hearing of His walking on water, raising Lazarus from the dead, feeding the 5,000 (from a couple of loaves and fishes), turning water into wine (a very popular party-goer) etc.
The last problem with a liar Jesus is why did he choose death over life, if he knew that he was lying? This is important because when Jesus was placed on trial he was given the opportunity to admit that he had lied and so escape death. He chose not to do this.
Someone who lived as Jesus did, taught the things he did, acted in the way he did and ultimately died as he did doesn't seem to come off as a liar.
So, if Jesus wasn't a liar, then was he a lunatic?