Does Jesus and Moses' so-called "Greatest Commandment" mean that it is our duty to develop ourselves to max?

you got it right. God is holy and he expects no less of us. Holiness means the absence of sin- even a single sin would make you unholy. That's the reason why Adam and Eve were banished for sinning just once. But then it has been proven during the Old Testament that it was impossible for man to perfectly obey the Ten Commandements- if it were possible, Jesus Christ did not have to come and save us. He showed us how to live a holy life- free from any sin. He demonstrated that He can do himself what he demanded of man. Because of his blood sacrifice, he redeemed us from the curse of sin thereby we are declared sinless because our sins were all paid for by his precious blood. Whereas, we are righteous in God's sight not because we did anything good ourselves but because God accepted Christ's sacrifice on our behalf.