In order to do that you must be greater than or equal to the thing you are trying to prove(ex: a mathematician prooving a math equation. The mathematician is greater than the math equation.).
First of all, define greater.
Let's say that greater means generally better: stronger, smarter, etc etc.
So, in order to prove something, something must be greater than the thing it is trying to prove. Well, a Christian would argue that the Bible proves God's existance. However, the Bible is just an object, it has no strength, no thoughts, etc, which would technically make God greater than the Bible. But to your logic, the Bible should be greater than God.
Ok, ok, so it's generally useless to compare a god to a book, an inanimate object, so let's say that the person proving existence with whatever tool means they are greater. But then, if the Bible does prove God's existence, man would be using it to do so. Would that make man greater than God? Then why worship God?
EDIT: Also, man couldn't be greater than God (if God existed exactly as Christians believe): man cannot create universes with magic power, among other things.
There are either two options to go for. Either Jesus was insane or He was indeed the Son of God.
He could have been lying.
Well, Jesus was seen performing miricles (many times). These are also in account.
There are accounts of early Egyptian magicians doing mind tricks that fit Jesus' miracles exactly: walking on water, turning water into wine, healing lepers, etc. Jesus could have very well been a creative trick magician.
So what if Jesus was impowered by Satan? First of all if this was true, then he probably wouldn't have cast out all those demons in people.
What happens if God isn't really all powerful; that he's good, but weak. What happens if, at the same time, there's an all powerfull evil god. What if Jesus was a trick to have man worship the wrong god?
EDIT: Assuming that the evil god would reward those who worship him.