J.Edwards, I came upon this article which may explain things better than I:
"While a peccable Christ may at first appear to be a more empathetic and human Jesus, a Christ who was able to sin is robbed of His full deity. Only an impeccable Christ measures up to the teaching of Scripture that the Son of God was fully God. Jesus could not sin because He was God. The term impeccable implies more than just the ability not to sin. Impeccable means not able to sin. Adam, before the fall, had the ability not to sin, but he was not impeccable because he had the ability to sin. Jesus was impeccable because as God He was not able to sin.


"Wait a minute!" you may say. "Isn't that one-sided logic? Wasn't it stated above that the Bible taught that Jesus was Man as well as God? Isn't it logical to conclude that, as Man, Jesus was capable of sinning?" While the God-side of the logic seems to be straightforward and conclusive, how is the man-side of the logic to be answered? Once again the God-side of the logic equation is as follows: Since Jesus is God and God cannot sin, then Jesus cannot sin. The man-side of the logic equation seems to contradict this conclusion. Since Jesus is man and man can sin, then Jesus can sin. What's the answer to this apparent contradiction in logic? The answer lies in the fact that there is a flaw in the man-side of the logic. The premise that man can sin is not 100% correct. A person does not have to sin or even be able to sin in order to be classified as man. The fact that we humans are all capable of sinning is not intrinsic to the definition of mankind. In our glorified bodies in the eternal future, for example, we will not sin or be able to sin (praise God!), but we will still be mankind. We are not going to be changed into angels or some new kind of creature--we will still be human and yet we will be sinless! To be capable of sinning is not a necessary part of what makes up the essence of man. So for Jesus to be fully Man, as the Bible teaches, does not necessitate His capability of sinning. Don't be thrown off by what appears to be sound logic. Even though every other person in history has been able to sin (in fact, not able not to sin!), this characteristic of mankind is not what defines the essence of man. However, for Jesus to be fully God as Scripture teaches does mean that Jesus could not sin. Here the logic is very sound and cannot be disputed.


One of the strongest Scriptural statements for the impeccability of Christ is found in Hebrews 13:8. "Jesus Christ is the same yesterday and today and forever." Can Jesus sin today? No! Will He be able to sin in the forever future? No! Well, what about yesterday? Is He the same or isn't He? If He was the same yesterday as He is today, then He could no more sin yesterday than He can today or forever. The constancy of Christ's character is demanded by Hebrews 13:8. The fact that Christ does not and can not change is known as the doctrine of immutability. Immutable means "not capable or not susceptible to change." The attribute of immutability applies just as much to the Person of the Son in the Trinity as the Persons of the Father and the Holy Spirit. In fact, a section of Psalm 102 which speaks of the unchangeableness of God is applied directly to Christ, the Son, in Hebrews 1:10-12. "But about the Son He says...'In the beginning, O Lord, you laid the foundations of the earth, and the heavens are the work of your hands. They will perish, but you remain; they will all wear out like a garment. You will roll them up like a robe; like a garment they will be changed. But you remain the same, and your years will never end.'" There was no "time out" period for the Son's immutability when He walked on this earth. Colossians 2:9 emphasizes that all the attributes of Deity were present in the Son during His earthly ministry. "For in Christ all the fullness of Deity dwells in bodily form." The attribute of immutability not only implies impeccability during the Son's earthly ministry, it demands it!


But what about the temptations of Christ? Doesn't the fact that Jesus was tempted imply that Jesus was "temptable?" No, not necessarily. Temptation is the attempt to ensnare and does not define the temptability of the target. A rowboat may attack another rowboat or it may attack a battleship. While the attack may be the same in both cases, the attempt in no way defines the vulnerability of the object attacked! In the case of our own bouts with temptation there may be an inward response without an outward "committing the act." This is because we have a sinful nature and are therefore temptable. As James 1:14 says, "Each one is tempted when he is carried away and enticed by his own lust." But the case of the Lord Jesus is unique. Without a sinful nature, the temptations of Christ were strictly a test from without with no inward response whatsoever. In terms of the illustration above, our Lord is like the battleship. Even if rammed by the rowboat, the attack causes no damage whatsoever.


Well then, if Jesus could not sin, are the temptations of Christ a sham? No! The temptations of the Lord Jesus were not to see if He would sin, but to show that He could not sin. Show whom? Well for one thing, Satan and his fallen angels. Satan did not necessarily know that Christ could not sin. He's not omniscient! As the wonder of the incarnation became evident to Satan, he was determined to thwart the purposes of God in any way he could. He tried on more than one occasion to attack the Lord Jesus with temptation. (See Luke 4:13.) But Satan learned to his undoing that the prophecy made in the Garden of Eden concerning the "Seed of the woman" could not be thwarted. (See Genesis 3:15.) The triumph of Christ over temptation demonstrated to all the angelic hosts, and to us as well, that the eternal Son of God was not susceptible to sin even though He became Man.


If Jesus could not sin, can He sympathize with us when we are tempted? Does the fact that Jesus was not able to sin make Him less "human" and less able to understand what we go through as fallen humans? Not according to Scripture. Hebrews 4:15 says that "We do not have a high priest who is unable to sympathize with our weaknesses, but we have one who has been tempted in every way, just as we are--yet without sin." This verse seems to imply that Jesus experienced the force of temptation the same way we do, but is that possible if Jesus could not sin? What is the answer to this apparently difficult question? One possible answer that has been suggested by Christians over the years is that Jesus may not have known that He could not sin. We do know that in the eternal counsels of God it was decreed that certain facts would not be in the conscious mind of the incarnate Son of God. As a Boy, He would learn facts about His own creation, and even as a Man He would be able to say that He did not know the "day or the hour" of His second advent. (See Mark 13:32 and Luke 2:52.) On the other hand, as God, our Lord may have known all along that He could not sin. The Lord's statement to the Pharisees in John 8:46 would imply that this was the case. In response to the Pharisees' argument against His deity, Jesus said, "Which one of you convicts Me of sin? If I speak truth, why do you not believe Me?" But even if Jesus knew He could not sin, He still felt the force of the temptations. Doesn't a tree feel the full force of a hurricane even though it doesn't fall? Consider an infinitely strong tree that cannot fail during a hurricane. Does this tree experience and feel the force of the storm as much as the trees that are more vulnerable? Of course it does. In fact, the tree that cannot fail experiences a greater force of the storm than a tree that falls.


What kind of a High Priest do we want, anyway? Do we want a High Priest who was so susceptible and vulnerable to sin when He lived on this earth that He almost sinned on certain occasions? Do we want a High Priest who had to struggle with His thought life before He overcame temptation in order to "feel" the force of the temptation the way we do? Do we want a High Priest who only had limited moral strength to resist temptation? Or do we want a High Priest who had infinite resistance against sin? Praise God, we have a High Priest who was not susceptible or vulnerable to sin in any way. He did not struggle to keep His thought life pure. He was the perfect Man. He was attacked but it was like a hurricane against the Rock of Gibraltar. He had infinite moral strength and therefore infinite resistance against sin--because He was God and could not sin. We have a High Priest who was the same yesterday as He is today and will be forever. The immutability of the eternal Son of God demands that He has always been impeccable."

David R. Reid