What kind of a sin is unbelief? not an obvious outward one, like murder or blasphemy, but a sin nonetheless, John 16:9, and the sinfulness of it can be seen in various different ways.
For one thing, it is disobedience, 2 Thess 1:8. God says, Believe, and the sinner says, No. God now commands all people everywhere to repent, but sinners cling to their sins. God calls us to turn to him with all our hearts, but we would rather run away in the opposite direction.
For another thing, it makes out that God is a liar (1 John 5:10). We are presented in the gospel with a Saviour who proclaims his mercy and truth side by side, but every aspect of the salvation he provides is met with and challenged and resisted. God says, ‘I am,’ and sinners say, There is no God. God says, I am angry with sinners every day, and sinners say, Because sentence is deferred, therefore I will fully set my heart to do evil. God says, I will pardon your iniquities, and the sinner says, I don’t really have iniquities as such, and anyway, this method of pardoning is too complicated. In short, sinners implicitly give more credit to claims that God is not merciful, not gracious, not forgiving, not determined to punish sin, than to God’s own claims to the contrary.
Thirdly there is the fact that nothing more could be done to bring the Saviour closer to sinners, Rom 10:6-8. He became a man, to save human beings. He came under the law, to redeem those that were under the law. In that he suffered, he is able also to succour them that are tempted. He is the bread of life for hungry souls, the water of life for thirsty souls, a robe of righteousness for unrighteous souls, the pearl of great price for destitute souls, and a door to life for dead souls with no access to the tree of life. He is a teacher for the ignorant soul, a healer for the sick soul, a guide and ruler for wayward and rebelling souls, a friend of publicans and sinners, a shepherd who delights to seek and to save those that are lost. There is nothing that a sinner needs, in short, which the Saviour cannot provide. In him are hid all the treasures of wisdom and knowledge – he is rich in mercy – he is made to us wisdom, and righteousness, and sanctification, and redemption – and unbelief wants nothing to do with him.
What else can show the sinfulness of unbelief. Nothing more could be done to make sin more unattractive (Rom 3:10-18). It is wicked in itself, and it is dishonouring to God, and it is damaging and ruinous to us. The consequences of it are plainly set out, and the warnings are written large. The right response, the rational response, the required response, is to flee from it, and from the wrath which will inevitably come on us for it, but unbelief does the opposite, and lingers and loiters and trifles. Nothing more, meanwhile, could be done to make the benefits of salvation more appealing (Psa 103:8-13). A pardon so full and so free, so unexpected and undeserved, so glorious and God-glorifying, so beneficial to us. The response of faith would be to yearn for these things and plead with the Saviour to save us with this salvation, and unbelief does the opposite.
See how outrageous the excuses were in the parable in Luke 14, when the man sent out the invitation, ‘Come, for all things are now ready.’ They all with one consent began to make excuse, and that’s only a small picture of how unreasonable and disrespectful it is for sinners to forsake their own mercy by turning away from such a suitable Saviour as God has provided. But although it can’t be dealt with by ourselves, it’s a sin which can be pardoned by the Saviour – there is power in the blood of Jesus Christ God’s Son to cleanse us from all sin, and the antidote to unbelief is the faith which he gives as a gift. ‘Let us therefore come boldly to the throne of grace, that we may obtain mercy, and find grace to help in time of need.’