Do not be unbelieving...
Following his resurrection, Jesus had appeared a couple times to His disciples, but one of the disciples, Thomas, was absent. When Thomas heard about Jesus' appearance, he didn't believe it. He said, "Unless I see in His hands the imprint of the nails, and put my finger into the place of the nails, and put my hand into His side, I will not believe" (John 20:25).
When Thomas said "I will not believe," was he sinning?
Well, on the one hand we could say Thomas' unbelief is only natural when the other disciples got to see Jesus in His resurrected body, but he didn't. The other disciples had hard evidence he was lacking. No wonder they believed.
We're not asking whether it's understandable, though, but whether it's wrong? And the answer has to be, "Yes, unbelief is sin." How do we know this?
We know this from Scripture. Jesus rebuked men for their unbelief. The Gospels of Matthew, Mark, and Luke all record Jesus saying, "You unbelieving and perverted generation, how long shall I be with you and put up with you?" Jesus warned Nicodemus, "...he who does not believe has been judged already, because he has not believed in the name of the only begotten Son of God" (John 3:18). And Hebrews warns us, "Take care, brethren, that there not be in any one of you an evil, unbelieving heart that falls away from the living God" (Hebrews 3:12).
It's surprising that unbelief is condemned because Scripture makes it so clear that belief and faith are gifts from God: "For by grace you have been saved through faith; and that not of yourselves, it is the gift of God..." (Ephesians 2:8). How can we be blamed for the lack of something God says is a gift from Him?
There are a number of ways this question could be answered, but the important thing is to recognize that unbelief is, in fact, sin. If God commands us to "take care" that we not fall away from Him due to "an evil, unbelieving heart," we are to obey this command. Jesus gave a similar command to Thomas: "Then (Jesus) said to Thomas, 'Reach here with your finger, and see My hands; and reach here your hand and put it into My side; and do not be unbelieving, but believing'" (John 20:27).
It's encouraging to me that Jesus commanded Thomas to believe because it's an indication that unbelief, like envy and greed and selfishness, is a sin that the People of God are tempted by and must fight against. With Thomas we will be tempted to declare, "I will not believe," but we must resist.
The first step in that battle is to recognize God's command that we believe, and to pray asking Him to give us what He commands. Remember the father who brought his demon-possessed son to Jesus to be healed? When the father approached Jesus, he expressed some hesitancy concerning whether or not Jesus could or would heal his son. Jesus answered him, "All things are possible to him who believes." The boy's father then responded with honesty and humility, "I do believe; help my unbelief." (Mark 9:24)
Recognizing how weak our faith is, we must join this boy's father and ask the Holy Spirit to help our unbelief, also.