Jesus rose early on the first day of the week, He appeared first to Mary Magdalene who went and told the rest. When they heard it they did not believe it. Afterward Jesus appeared to two of them while they were walking in the country. They returned and reported it to the others but they did not believe them either. Later Jesus appeared to the eleven and rebuked them for their lack of faith and stubborn refusal to believe. (Mark 16:9-14 NIV)
Unbelief is the unforgivable sin, because only it cannot be atoned for. How can unbelief be atoned for when we refuse to believe we can be atoned for? People die without Christ all the time and spend eternity separated from Him all because of unbelief. Here the disciples were told on several occasions that Christ was raised from the dead. But yet they did not believe it. It wasn’t just Thomas who suffered from unbelief. All the disciples suffered from it. Even today can we truthfully say that we do not? We can believe Christ has risen because God’s word gives us the proof. Remember much of the New Testament was written by eye witnesses to Christ Resurrection. They verify to what they had seen and tell us that we can rest assured all that is written is true. More than that, we have the Holy Spirit testifying the truth in our hearts. If he is living within us we can testify to the truth of Christ resurrection. You ask me how I know He lives, He lives within my heart.