Both God and man
There has to be some way of overcoming disobedience and rebellion. The one who overcomes this rebellion must be both God and man. He has to be man, otherwise he could not act in our name. If, for example, you are arrested for speeding, I cannot go into the court and say to the judge, 'I will take his place and his punishment.' The judge will say, 'You have nothing to do with this case.' So he who comes to atone for our rebellion must be human to act in our name. He must also be God in order that his actions will have infinite value, in order that he can redeem us.