Seeing Is Not Always Believing

Abraham got up and left his home, his family and everything familiar in order to follow a promise for which God had given no evidence that it would ever be fulfilled. Abraham is held out as an example of the behaviour / faith that God wants from all whom He has called to be part of His spiritual family.

Israel on the other hand, is held out as an example of behaviour / lack of faith that took place in the midst of overwhelming evidence that God was in their midst. 

During Jesus' ministry, physical proof that our Creator had 'pitched His tent among us' was also overwhelming. Yet the people demanding more proof.

Atheists say that if Jesus would only appear in front of them like He did with the disciples of His day, then they would believe. 

No they wouldn't. 

Atheists are naturalists. If Jesus suddenly appeared in front of them, they would come up with a naturalist explanation for what 'they thought they'd seen.'
Atheists aren't lying when they say they would believe. They simply don't understand who they are.