Unbelief is just another symptom of sin

Why do atheists not believe in God? Why do Christians sometimes struggle with doubt in God's promises?

Unbelief is only one of the whole string of symptoms of the disease called SIN. When we have the flu, we cough. It is natural. Likewise, sinners are unbelieving by nature. Adam and Eve first sinned by rejecting to believe in the truth of God's word "Eat of the tree and you will die", and thus brought slavery to sin and death to all mankind. All mankind from Adam and Eve are thus all bound to do the same that their parents did: disbelieve in God. It is a natural fruit of a diseased tree called humanity.

But the Good News is that the Son of God came to the world as a Man. He lived, and He was faithful to God's word. He trusted in God with His destiny, even as He undertook the wrath of God upon himself and died on the Cross for our sin. Even though He was terrified, and He knew that He was going to undergo the judgement of God, He trusted in the Father, that He would raise Him from the dead, and restore the glory that He had at first. He was not faithless, but gave glory to God as He gave Himself to be mocked and tortured by sinful men.

And when Jesus Christ saves a sinner, He imparts onto him a measure of His faith by the Holy Spirit, so that the sinner may believe in God through Himself. He is healed of his faithlessness, and he becomes faithful. He begins to see the evil of unbelief, the monstrosity of likening God to a sinful, unreliable creature that cannot be trusted. God works in the Christian's heart, so that not only he believes in God, but so that he himself may become more and more a trustworthy man himself - so that in this world of scepticism and distrust he may become the salt and the light, that the world may be told that it is okay to believe again.

Matthew 17:17 - And Jesus answered and said, "You unbelieving and perverted generation, how long shall I be with you? How long shall I put up with you? Bring him here to Me."