Then why is there still evil and sin? And how can he rationally have erased PAST evil and sin?
Answers to your 2 questions:
1. There is still evil and sin because God has given us free will, and he is unwilling to take this away. He wants us to have the opportunity to choose. That is part of the joy of being God's creation. The difficulty you are having is in seeing that Jesus' work is not done. There is the first coming, and then the second coming. In between, the Holy Spirit is doing its work in the hearts of men in order to save as many as possible (all of them who are willing to repent) before the second coming. At the second coming, Christ slays all those who are unwilling to accept the truth. Then there will be no sin, because it will have been slain. Simple as pie.
2. Erasing and cleansing are not the same thing, in my view. Cleansing means it is taken off of you, and it falls somewhere else (away from you and God). But none of this matters, because God is also the creator in Abrahamic religions, and that which gives life can also take it away. Just as he can bring about things out of nothingness, he can also destroy them eternally. I know that you probably don't believe in the idea of creating something out of nothing, seeing as you are an atheist. Consider this: have you ever heard of a prisoner's dilemma? Or perhaps have you heard of the economic concept of benefits of trade? In both of these examples, wealth is created simply by loving other people and seeing how you depend upon each other, and they are well documented and proven both by mathematicians and economists. Of course, again, you probably don't believe in this, because most atheists believe completely in our rudimentary understanding of physics. Check out the Bose-Einstein Condensate. It's a fourth state of matter, theorized by Einstein and proven by Bose. It behaves in ways that physicists cannot yet explain. The atoms share energy, and behave as a community as opposed to as individuals.
Is it because it gives us a perfect moral example to follow (Jesus) and if we follow that example we are foregiven? And if so, could some of us not act in such a way without said example? Also, if this is the idea, then why would he have to suffer on the cross after giving the example and not simply ascend into heaven without that?
Answers to your questions in order:
1. Yes. We are told to follow Jesus' (pbuh) example, plain and simple.
2. We have the free will to act in this way. Unfortunately, it seems like human-beings since the original sin of Adam/Eve have been unable to remember the happiness and ultimate freedom that comes from loving others as yourself (the example of Jesus). This is tied to our fear of death. It began as shame, and ended as death. There is a great quote from Anthony Hopkins in the movie "The Edge:" "Do you know what people die of when they get lost in the woods? No? They die of shame. They are so ashamed that they have gotten lost, that they just sit down and die." Jesus was able to show us that even death is meaningless via the resurrection. In this way, he destroyed our reason for staying ashamed, and showed us that there is no reason to live in shame and fear. Without this proof, apparently mankind would have found another reason to keep on sinning regardless of what philosophers and prophets kept on telling them--fear of death.
3. He suffered on the cross so that we could see that suffering is temporary; that death is temporary. He chose to suffer. That's the point people don't understand. He chose to suffer because he wanted to do God's will in showing stubborn humanity that even death is meaningless. It was an act of love, both of God's love toward us, and Jesus' love toward God, and through the connection of the two of them, both in both. And, to anticipate your response, God was not turning his back on Jesus. After the resurrection, Jesus was rewarded more than before by God for his faithfulness in the face of torture, and he now sits on the right hand of God.
Peace.
