To answer the question fully, you have to first place yourselves in the position of the Jews of the time of Jesus. The Pharisees (teachers of the Law for the time) held very strict religious traditions based primarily off of what we know as the Old Testament as well as man-made traditions that strictly defined in very lawyer-like fashion what a person can do and when they can do it as well as how to go about doing it. These legal traditions were the widely held beliefs of the time when Christ was born to such a degree that when Christ came preaching and teaching the Law in a way that they had not been used to hearing it, many were amazed. In addition, throughout Scripture, the people stated that he "taught as one with authority". The primary belief concerning the Messiah of the time was that He would come, ransom Israel from captive Rome and establish a new throne in the line of David and many men attempted to do this along the way. When Jesus came teaching the "radical" teachings and doing things in His own authority, it's bound to tick off the religious elite. When he did not oust Herod or Caesar from the throne and instead died, this gave the impression that he was not the Messiah-to-be but their error was the belief that His kingdom would be an earthly kingdom instead of a heavenly one.
True that all of Jesus' followers deserted Him at the time of His crucifixion, 11 were redeemed after his resurrection from the dead and they were eye-witnesses to this. They saw him die, and John wrote that blood mixed with water came out of His body when he was pierced by the spear indicating that he was indeed dead. After seeing Him resurrected, they were taught many things by Him for 40 days following and then saw Him taken up to Heaven.
Now for the disciples to teach the resurrection of Christ meant that they were either liars or men of truth. Also know that by teaching the resurrection, they would be putting their lives on the line to be ostracized from the community at large, imprisoned and killed for their message because Christ, according to many of the Jews, was a blasphemer who claimed to be God and anyone siding with him should be killed. No man would stand by a lie to the point of death. They were commissioned by their Lord and Savior even to the point of death to preach the resurrection and the remission of sin if others would put their lives on the line and believe and trust in Christ.
There's a great book by Lee Strobel called "The Case for Christ" who touches very indepth on these issues.
@Putin33 - Jesus clearly stated that the most grave of sins was to blaspheme the Holy Spirit, or in essence, reject Christ as Savior. He said that a man can be forgiven of anything except this.