There are some pretty atrocious sins out there, but, without exception, they're all forgivable. Christians ultimately believe that no sin is beyond God's forgiveness and, by recognizing our utter dependence upon Him, we are invited to always present our sins before the Lord of All and to become reconciled with Him. If not, then we must admit to the heresy of Pelagianism―the idea that we can "earn" our way into Heaven with good works.
(By the way, of all the religions and pseudo-religions in the world, there are exactly only two religions in the world that teach that one can't earn one's way into Heaven: Judaism and Christianity. Everyone else believes they're on the brownie point system and that a deficit in one area can be made up with some applied vigor in another area.)
God reminds us continually in the Bible that all we need do is throw our decrepit selves onto Him and He will forgive us.
He tells us that He will remove our sins as "far as the east is from the west." (Psalm 103:10-12).
Considering the seeming limitless nature of the universe, this is a substantial assurance on God's part.
God assures us not to worry as He has cast all of our sins behind our backs and thus won't be counted against us. (Isaiah 38:17)
The Prophet Micah tells us that God will crush our sins underfoot and will cast our sins into the deepest part of the deepest ocean. (Micah 7:19) Out of sight, out of mind―God's Mind.
The Prophet Isaiah 43:25 assures us that God will blot out our sins. (Isaiah 43:25) He removes them from His Own Memory! Even He refuses to remember them!
Christ tells us that if we want to be forgiven, we should in turn forgive others. When you think about it, it's a pretty good deal. We couldn't have hoped for a fairer shake from anyone let alone from the Lord of All. How can we know He's good for His word? He died for us. How many of the rest of us would literally put our lives on the line for the sake of a bunch of misfits like us? Jesus' blood washes us and makes us pure as the driven snow. (1 Jn 1:7)
But, despite these assurances, there is, however, a sin that destroys this understanding of ourselves, of God and of the dependent relationship between He and us. Though it's understandable that some would think so, the Unforgivable Sin is not suicide. Rather, it's intentionally shutting God out of our lives and refusing to have hope. We are immortal beings endowed by our Creator, in Whose loving image we are made, with hope and a vision of our future life in the next world. The Sin of Hopelessness, also called "The Unforgivable Sin," is often associated with suicide not because suicide is unforgivable but rather because it's associated with a sense of despair that often results in either suicide, homicide or other sins.
To be specific, the sin of despair is not a mere sadness or sense of gloom but rather the total loss of existential hope. It's a sin because it contravenes our basic understanding of God and His compassion, His omnibenevolence and His faith-keeping. It's not necessarily a sin committed out of hatred of God (i.e., misotheism) or even because of one's personal selfishness. Despite this, despair still has a power to cause harm to the human soul because it specifically denies the sole cure for sin—God's love and forgiveness. Without God's love and forgiveness, we are truly all doomed. If we deny God's ability forgive us and deny his willingness to do so and refuse to accept the love and forgiveness He gladly offers us, then there is truly no hope for us. At that point, we could fall prey to a great number of other sins.
In the Gospel of Matthew, we find Christ's parable about the Three Servants. The Savior describes Heaven as a rich person who leaves his estate in the care of three servants giving them money in proportion to their skill and trustworthiness. To one, he gives 5000 gold coins, to another 2000 and to the last 1,000. The first two invest the money they were given doubling their investments. The least trusted servant decides to dig a hole and bury the coins he was given. When the rich man returns, he is grateful to the first two servants who managed their money wisely and gives them many honors, better jobs and a share in the master's happiness. He reacts less generously to the third, more "careful" servant:
Then the servant who had received one thousand coins came in and said, 'Sir, I know you are a hard man; you reap harvests where you did not plant and you gather crops where you did not scatter seed. I was afraid, so I went off and hid your money in the ground. Look! Here is what belongs to you.' 'You bad and lazy servant!' his master said. 'You knew, did you, that I reap harvests where I did not plant and gather crops where I did not scatter seed? Well, then, you should have deposited my money in the bank and I would have received it all back with interest when I returned. Now, take the money away from him and give it to the one who has ten thousand coins. For to every person who has something, even more will be given and he will have more than enough; but the person who has nothing, even the little that he has will be taken away from him. As for this useless servant—throw him outside in the darkness; there he will cry and gnash his teeth.' (Mat 25:24-30)
As human beings, we are given incredible gifts of existence and God's faithful love. If He had wanted to, He could have simply chosen to have not created us. Like the three servants in the parable, we were given this precious gift called life in which we must invest and then return to its Creator. Without trusting in Him, we would never have the confidence to approach Him to ask for forgiveness. Judas became despondent when the enormity of his sin, betraying Jesus for thirty pieces of silver, overtook him. He ultimately hanged himself because he refused to approach God in prayer and ask for forgiveness. Thus, his sins were compounded by his refusal to ask for forgiveness —a truly unforgivable sin because it denies God's ability to forgive. It is the ultimate Catch-22 situation. Peter, on the other hand, abandoned and denied Christ even after being clearly forewarned. (Mark 14:72) But Peter was blest with the understanding that Christ was his spes unica, his "only hope," and so returned to God asking for forgiveness.
In addition to the sin of despair itself, there is an additional, secret hidden possible sin inherent in the sin of despair that must be seriously considered: specifically, heresy. It would be heresy to suggest that a human being could commit a sin so horrible that even God couldn't forgive it. It would be contrary to Christianity to suggest that God was unwilling or incapable of supplying us with what's needed for salvation.
Remedies against Despair
God can't forgive us unless we believe He can forgive us:
Listen to these words, fellow Israelites! Jesus of Nazareth was a man whose divine authority was clearly proven to you by all the miracles and wonders which God performed through Him. You yourselves know this, for it happened here among you. In accordance with His own plan God had already decided that Jesus would be handed over to you; and you killed Him by letting sinful men crucify Him. But God raised Him from death, setting Him free from its power, because it was impossible that death should hold Him prisoner. For David said about Him, 'I saw the Lord before me at all times; He is near me and I will not be troubled. And so I am filled with gladness and my words are full of joy. And I, mortal though I am, will rest assured in hope, because You will not abandon me in the world of the dead; You will not allow Your faithful servant to rot in the grave. You have shown me the paths that lead to life and Your presence will fill me with joy. (Acts 2:22-28)
It comes down to the realization and acceptance that if God can forgive us, then we should be able to forgive ourselves:
Now that we have been put right with God through faith, we have peace with God through our Lord Jesus Christ. He has brought us by faith into this experience of God's grace, in which we now live. And so we boast of the hope we have of sharing God's glory! We also boast of our troubles, because we know that trouble produces endurance, endurance brings God's approval and his approval creates hope. This hope does not disappoint us, for God has poured out his love into our hearts by means of the Holy Spirit, who is God's gift to us. For when we were still helpless, Christ died for the wicked at the time that God chose. (Rom 5:1-6)
The knowledge that God is ready, willing and able to forgive our sins has brought countless billions to Him over the past 3,300 years.
It's a particular comfort to Christians to know that, via the Sacrament of Penance, we're allowed a great number of mulligans―that is, if we take advantage of them and don't presume upon God's generosity in regards to our lackadaisicalness and lack of commitment. To be forgiven, we must first recognize that we are sinful. We must then be desirous of His forgiveness. We must agree to try our hardest to avoid sinning and, in fact, the near occasion of sin and especially any instance or situation that would otherwise weigh us down, in the future. But, first and foremost, we must never give up on God.
God isn’t merely the "Forgiver-in-Chief" as is if He were at our beck and call. He is Love Itself and desires our salvation. After all, He didn’t create death nor does He seek to destroy the living. Instead, He created all things that they might be. There isn’t any poisonous destruction in the good He created. (Wisdom 1:12-14)
The admonition to "Be not afraid!" appears 365 times in the Bible―the same number as there are days in the average year. Sometimes God speaks these words to humans. Sometimes, an angel is His mouthpiece.
We might ask, why so many times? Perhaps it's because He desires that we love Him. But even if all our sins are as scarlet, all He asks is that we not abandon hope. For with hope, we may yet rest in Him.