A friend recently asked how do Christians know that Christ is sinless. Someone recently told this friend that Christ was fully God and fully man, so therefore Christ sinned since “all have sinned and fall short of the glory of God” (Romans 3:23). Of course in this case, Jesus is the exception. Often times when we say “all” we exclude in a natural manner those not in the category, for instance no one naturally take it that this means all things in the universe sin. We don't take it that all rocks are sinful. This would be a category that would be excluded, even though a rock can be used for sinful purposes such as raw material for making an idol or it could be used as a murder weapon. In these cases, still we do not hold the rock as sinning but merely an object used for the purpose of sin. On the other end of the spectrum, we don't count the angels in heaven nor God himself as sinful. So the all excludes many things. The problem comes in since we would naturally count that the all in this since are all humans.
Since Jesus Christ is human, then should we not count him as a part of this category? Well looking at the category further, when we say that Adam was a sinner, we do not mean that he was a sinner prior to him eating the fruit of the tree of the knowledge of good and evil. Prior to that time, he was innocent. In reality, it is not the fact that someone is a sinner because he is human, but the fact that he or she is a child of Adam, the first man to sin and the one who passed sin down to all his children. Adam sinned and all those after him who were born from him are sinners. So it is slightly off to say that all humans are sinners, but it is more correct to say that all the offspring of Adam are sinners. While Jesus is human, he is not a son of Adam, but came to be in human form in a miraculous birth, pointing to whom is his true father, that is God the Father.
We can find many verses that show us the sinlessness of Jesus.
15For we do not have a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tempted as we are, yet without sin.
26For it was indeed fitting that we should have such a high priest, holy, innocent, unstained, separated from sinners, and exalted above the heavens.
9And they made his grave with the wicked and with a rich man in his death, although he had done no violence, and there was no deceit in his mouth.
46Which one of you convicts me of sin? If I tell the truth, why do you not believe me?
II Corinthians 5:21:
21For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.
I Peter 2:22:
22He committed no sin, neither was deceit found in his mouth.
I John 3:5:
5You know that he appeared to take away sins, and in him there is no sin.