When we speak of Christ's sinlessness we generally refer to His humanity. It is unnecessary to plead the sinlessness of Christ's deity, as deity by our definition cannot and does not sin. The doctrine of Christ's sinlessness has been free of any fundamental controversy. Even the most crass heretics in history have not denied this of Christ.
The sinlessness of Christ does not merely serve as an example to us. It is fundamental and necessary for our salvation. Had Christ not been the "lamb without blemish" He not only could not have secured anyone's salvation, but would have needed a savior Himself. The multiple sins Christ bore on the cross required a perfect sacrifice. That sacrifice had to be made by one who was sinless.
Christ's sinlessness had negative and positive aspects to it. Negatively, Christ was completely free of any transgression. He broke none of God's holy law. He scrupulously obeyed whatsoever God commanded. Despite His sinlessness, Christ even obeyed Jewish law, submitting to circumcision, baptism, and perhaps even the system of animal sacrifice. Positively, Christ was eager to obey the law; He was committed to doing the will of His Father. It was said of Him that zeal for His Father's house consumed Him (John 2:17) and that His meat was to do the will of His Father (John 4:34).
One difficulty concerning the sinlessness of Christ is related to Hebrews 4:15: "For we do not have a High Priest who cannot sympathize with our weaknesses, but was in all points tempted as we are, yet without sin." If Christ was tempted as we are, how could He have been sinless? The problem becomes even greater when we read James 1:14-15: "But each one is tempted when he is drawn away by his own desires and enticed. Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death."
James describes a kind of temptation that arises from sinful desires within us. These desires are already sinful in nature. If Jesus was tempted as we are tempted it would seem to suggest that He had sinful desires. Yet this is precisely the point of the qualifier "yet without sin" in the book of Hebrews. Jesus had desires. But he had no sinful desires. When He was tempted by Satan the assault came from the outside. It was an external temptation. Satan tried to entice Jesus to eat during His period of fasting. Jesus surely had physical hunger; He had a desire for food. Yet there was no sin in being hungry. All things being equal, Jesus wanted to eat. But all things were not equal. Jesus was committed to obeying the will of the Father. He had no desire to sin.
It was by His sinlessness that Jesus qualified Himself as the perfect sacrifice for our sins. However, our salvation requires two aspects of redemption. It was not only necessary for Jesus to be our substitute and receive the punishment due for our sins; He also had to fulfill the law of God perfectly to secure the merit necessary for us to receive the blessings of God's covenant. Jesus not only died as the perfect for the imperfect, the sinless for the sinful, but He lived the life of perfect obedience required for our salvation.