From Anstey’s Doctrinal Definitions:
This refers to the Lord Jesus Christ having a human nature that is incapable of sinning. It does not exactly touch on the question of whether He sinned in His life (which, of course, He didn’t), but whether He had a nature that was capable of sinning. While all Christians unanimously agree that Christ did not sin, many think that He could have sinned, if He chose to. But this false idea strikes at the impeccability of Christ’s Person and is a serious error touching the doctrine of Christ.
When Christ came into the world (His incarnation), He took manhood (a human spirit, a human soul, and a human body) into union with His Person. This union of the divine and human natures is inscrutable (Matt. 11:27). In doing so, He did not take the innocent human nature that Adam had before he fell. That nature was without sin, but it didn't have the knowledge of good and evil, and it was capable of sinning—which Adam sadly demonstrated (Rom. 5:12). Christ couldn't have taken that nature because it no longer existed in its innocent state at the time of His coming into the world. It had been corrupted by Adam's disobedience and was fallen. Nor could Christ have taken into union with Himself that nature in its fallen state, for in doing so, He would have taken sin into His Person, and thus He would cease to be holy. Had He done that, He would have ceased to be God, because holiness (the absence of evil) is one of the essential attributes of deity! (Isa. 6:3; Rev. 4:8) The Bible indicates that God prepared for Him a "holy" humanity—spirit, soul, and body (Luke 1:35; Heb. 7:26; 10:5). Being holy, the Lord Jesus had a human nature that could not sin.
Now, since Adam's fall, when we speak of a person sinning—regardless of who it might be—it immediately brings into the discussion the person having the sin-nature that would produce those sins. Sins, as we know, are the product of sin (the nature). Hence, to say that the Lord Jesus could sin (though He didn't) implies that He had the fallen sin-nature! This is a terribly mistaken assumption which the Word of God most surely does not support.
The following references show that Christ did not partake in fallen humanity, though He most surely became a Man:
1 John 3:5 says, "In Him is no sin." This single statement from the Word of God settles the question as to whether Christ could sin. It tells us that He didn't have the sin-nature in Him, and therefore, He couldn't possibly commit sins.
In Luke 1:35, in connection with the Lord's incarnation, the angel that came to Mary said, "That holy thing which shall be born unto thee shall be called the Son of God." This tells us that the essence of His nature as a Man is "holy." This could not be said of any other man. We were not born holy (Psa. 51:5).
In Luke 3:23, when tracing the Lord's lineage down from Adam, Scripture says, "Jesus Himself began to be about thirty years of age, being (as was supposed) the son of Joseph." The phrase "as was supposed" is inserted in the text by the Holy Spirit to show that the Lord was not the natural son of Joseph; He was only his legal son. He was "conceived" by the Holy Spirit, and not through Joseph (Matt. 1:20). The fact that Scripture notes that Joseph had nothing to do (biologically) with the Lord's conception, shows the care that God takes in guarding against any thought that Christ inherited the fallen sin-nature by having it passed down to Him through the descendants of Adam.
Romans 8:3 says, "God sending His own Son in the likeness of sinful flesh, and for sin condemned sin in the flesh." Here again we see that Scripture is careful in guarding Christ's humanity, stating that His coming into Manhood was "in the likeness of sinful flesh." Thus, He did not have "sinful flesh," but was only in the "likeness" of it. That is, from all outward appearances, He looked like any other man (Heb. 10:20), but inwardly, He did not have the sin-nature.
Hebrews 2:6 says, “What is man that Thou art mindful of him?” This is a quote from Psalm 8. The Psalmist wonders at the grace of God that would take up with men. The word here for “man” in the Hebrew is “Enosh.” It denotes man’s weak, frail state—implying a fallen and degenerated condition. The psalm goes on to say, “ ... Or the son of man that Thou visitest him.” This refers to God's visit to the human race in the Person of His Son (Luke 1:78). Note: on this occasion, the Psalmist uses a different word for “man” in the Hebrew from what he had previously used. Here it is “Adam,” which does not carry the connotations of “Enosh.” This means that when Christ would visit mankind, in becoming a Man, it would not be in the degenerated “Enosh” state.
Hebrews 2:14 says, "Since therefore the children partake of blood and flesh, He also, in like manner, took part in the same." Here again, Scripture carefully guards the sinlessness of Christ's humanity. Again, it uses two different words in the Greek to distinguish between fallen men taking part in humanity and Christ taking part in humanity. The first word (koinoneo), translated "partake," refers to a full, common sharing in something. It is used in this verse to denote the kind of sharing in manhood that all in Adam's race have. Since it is a full sharing, it would necessarily include partaking in the fallen sin-nature. The other word (metecho) is translated "took part," which means to take part in something without specifying how far the partaking went, is used to denote the sharing that Christ had in humanity. He took part in humanity, but not to the point of partaking in the fallen sin-nature, which all other men have. (See the footnote in J. N. Darby's translation on this verse.)
In Hebrews 4:15, regarding the Lord's testing and temptations in His earthly pathway, the writer says, "We have not an high priest which cannot be touched with the feelings of our infirmities; but was in all points tested like as we are, yet without sin [sin apart]." Unfortunately, reading this verse as it is in the KJV (and in many modern translations), it looks as though it is saying that the Lord didn't commit any sins in His life. But this is not the point in the verse. The phrase "yet without sin," should be translated "sin apart." Sin apart means that His temptations were not in the classification of temptations having to do with the sin-nature. There are two classes of temptations to which men are subjected: there are outward temptations and testings (holy trials) whereby one's faith and patience are tested, and there are inward temptations that result from having a sin-nature (unholy trials). (See James 1:2-12 and James 1:13-16.) The writer of Hebrews is simply stating that the Lord was tested in every way that a righteous man could be tested but not in the class of temptations that are connected with the indwelling sin-nature. The reason for this is obvious—He didn't have a sin-nature. J. N. Darby said, "There are two kinds of temptations: one is from without, all the difficulties of Christian life; Christ went through them and He has gone through more than any of us; but the other kind of temptation is when a man is drawn away of his own lust, and enticed. Christ, of course, never had that" (Notes and Jottings, p. 6).
In John 8:46 the Lord said to His detractors, "Which of you convinceth Me of sin?" No one could prove that He had that fallen nature, because no one could point to a single sin that He had committed.
In John 14:30, the Lord announced to His disciples, "The prince of this world cometh, and hath nothing in Me." He was referring to Satan's coming to harass and terrify Him, but He assured them that there was nothing "in" Him (i.e. the sin-nature) that would respond to his attacks.
James 1:13 says that "God cannot be tempted with evil." Thus, holiness is an intrinsic attribute of God (Isa. 6:3; Rev. 4:8). If, when God in the Person of His Son became a Man (1 Tim. 3:16), He thereby became capable of being tempted to do evil, then He relinquished one of His essential attributes in deity. Hence, if the doctrine that Christ could have sinned is true, then Christ ceased to be all that He was as God in becoming a Man! This is blasphemy!
1 John 3:9 says, "Whosoever is born of God doth not commit sin; for His seed remaineth in him: and he cannot sin, because he is born of God." This verse is speaking about the believer having a new nature (resulting from new birth) which cannot sin. John explains that this is so because, being begotten of God, we have "His seed" in us. It confirms what every Christian knows already—that God's "seed" (or life) cannot sin. Building on this fact, since Christ is "God manifest in flesh" (1 Tim. 3:16), then it naturally follows that He could not sin—because God cannot sin! What could be more clear than this?
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There are three main objections to this truth concerning Christ’s humanity:
1) Those who hold that Christ could have sinned, but didn't, think that it is robbing the Lord of His glory of obedience to say that He couldn't sin. They say that if this were true, then Christ would get no credit (thus no glory) for His life of perfect obedience to His Father, because He couldn't do anything but what was right.
To human reason it might look as though these things concerning Christ's Manhood are robbing Him of glory, but really to teach that He could sin attacks the impeccability of His Person and sullies His glory. We are not wiser than the Word of God. When our human reason leads us to conclusions that are in collision with Scripture—which this doctrine does—then we must lay down our thoughts and accept what Scripture says as the final authority, for it is God's infallible Word (Psa. 12:6; John 10:35).
2) Those who hold that Christ could have sinned, but didn't, point to the temptations of the Lord in the wilderness, and ask, "What was the purpose in having Christ pass through those temptations when He couldn't fail?" The answer is that they were not for God to find out whether Christ would or would not sin. He knew His sinless perfection and pronounced His approval upon Him—"This is My beloved Son, in whom I am well pleased"—before He was tempted (Matt. 3:17–4:11). If the temptations were for the purpose of discovering whether or not Christ would sin, then God's pronouncement would have been after He went through the testing. It would be, so to speak, His "stamp of approval" on Christ's perfect obedience. But these temptations were not for God; they are for us to see and know, beyond any shadow of doubt, that Christ couldn't sin. If He had any tendency in Him to sin whatsoever, it would have come out under such intense testing—but there came forth nothing but moral perfection.
Where many are confused as to this, is in thinking that "tempted" (Matt. 4:1) loses its meaning if it doesn't involve the possibility of sinning. But this is a mistake. Tempt means to test, and not all tests imply the possibility of failure. Suppose there was a valuable object made of fine gold—it was 100% pure gold. But its genuineness was disputed. So, to prove what we already know, we have it tested by a jeweller. And sure enough, it comes back certified as being 100% pure gold. Why was the object tested? We knew what it was made of all along. Obviously, the test was for those who had some question about it. Likewise, with the temptations of the Lord, all such testing only proved what was true of Him—that He could not sin. Those tests which the Lord underwent are recorded in Scripture for us so that we would know this blessed fact concerning the Son of God.
3) Some who hold that Christ could have sinned, but didn't, say that there could be no true manhood without a person having the capability of sinning, because it is an essential characteristic of being a human. They will say that teaching that Christ couldn’t sin is saying that He was not a real man. They believe that teaching this robs Christ of the ability to sympathize with us in our temptations of lust and sin. However, the truth is that there are many things that we experience in life as men that the Lord never experienced, but this does not mean that He wasn't a real Man. Nor does it disqualify Him from being a sympathetic High Priest. For instance, we experience infirmities (sicknesses), but the Lord never did. We have experienced the joy of forgiveness, but the Lord never needed forgiveness. He never married, nor fathered children, yet He was (and is) a real Man. Why would we think that He had to have all these experiences before He could be considered as being a real Man?
Hebrews 4:15 is often quoted to support this mistaken idea. It says that the Lord was tempted "in all points like as we are," which (in their minds) would include the temptation to sin. However, those who say such things have overlooked the fact that the inspired writer qualifies these temptations by saying, "sin apart"—that is, such temptations pertaining to sin were of a class apart. This means that the Lord's temptations were not those in that category. Had they read this verse more carefully, they also would have seen that the writer is referring to trials in connection with our "infirmities," which are bodily sicknesses (Matt. 8:17; John 5:5; Rom. 8:26; 2 Cor. 12:5). Infirmities are not temptations to sin. Let us note, too, that even though the Lord did not have infirmities personally (He was never sick), the verse says that He is able to sympathize with our infirmities. This shows that it is a false assumption that Christ could not have been a true man without experiencing everything that we experience.
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The reasoning in these blasphemous ideas is absurd. Think of the ramifications of Christ being able to sin. If He could sin when He was on earth, He could sin now in heaven—for Scripture says that He is "the same yesterday, and today, and forever!" (Heb. 13:8; Acts 1:11) Far be the thought, but if He were to sin now, He would surely be expelled from heaven, as Satan once was! And what would happen to us? We would lose everything—our Saviour, our salvation, and all our blessings—because everything we have is "in Christ!" If this bad doctrine were true, then we are not eternally secure, as Scripture teaches (John 10:27-28), because at any moment Christ could sin and we would lose our Saviour. Furthermore, if Christ did sin, what part of Him would go to Hell? Because in His incarnation, there was a union of the human and divine natures that can never be dissolved.