*"We confess ,then, that He assumed all the natural and innocent passions of man. For He assumed the whole man and all man’s attributes save sin. For that is not natural, nor is it implanted in us by the Creator, but arises voluntarily in our mode of life as the result of a further implantation by the devil, though it cannot prevail over us by force. For the natural and innocent passions are those which are not in our power, but which have entered into the life of man owing to the condemnation by reason of the transgression; such as hunger, thirst, weariness, labour, the tears, the corruption, the shrinking from death, the fear, the agony with the bloody sweat, the succour at the hands of angels because of the weakness of the nature, and other such like passions which belong by nature to every man.
All, then, He assumed that He might sanctify all. He was tried and overcame in order that He might prepare victory for us and give to nature power to overcome its antagonist, in order that nature which was overcome of old might overcome its former conqueror by the very weapons wherewith it had itself been overcome.
The wicked one , then, made his assault from without, not by thoughts prompted inwardly, just as it was with Adam. For it was not by inward thoughts, but by the serpent that Adam was assailed. But the Lord repulsed the assault and dispelled it like vapour, in order that the passions which assailed him and were overcome might be easily subdued by us, and that the new Adam should save the old.
Of a truth our natural passions were in harmony with nature and above nature in Christ. For they were stirred in Him after a natural manner when He permitted the flesh to suffer what was proper to it: but they were above nature because that which was natural did not in the Lord assume command over the will. For no compulsion is contemplated in Him but all is voluntary. For it was with His will that He hungered and thirsted and feared and died."*
This is from An Exposition of the Orthodox Faith (Book III), Chapter 20, by John of Damascus. To summarize: Christ had two natures and so he was fully man and fully God, and He did not assume the attribute of sin. Man was not given the ability to sin when he was created, he was made perfect. That is why the assault on the nature of man was an outward assault in the form a serpent, not because of an implanted thought. Likewise, Jesus was assaulted by Satan because he did not have in his nature the ability to sin, and the assault was not a test of His will, but it was an attempt to disrupt Gods plan for our salvation. For by his fortitude, Jesus shows us that he has overcome sin, and that we can just as easily overcome it, and by his dying he has overcome death, and promises that we too shall overcome it.
Furthermore, I can’t agree with those that say he could sin, because then we fall into the trap of saying that Jesus and God are not one. For how can he be of two seperate natures in will, and be one, if the one nature is capable of sin and the other is not? The two are not separated not even in will. Jesus professed that he came to do His Father’s will (John 6:38), not meaning that he had a separate will of his own, but he and the Father are one (John 10:30). If he is always doing his father’s will, is it the will of the father that he might sin? I do not believe so.