I believe so strongly in Jesus Christ because of the truth found about Him in the Bible. As the Son of God, Jesus Christ came into the world to save sinners - by qualifying to take God's wrath in their place through a sinless life, by voluntarily laying down His life and dying for our sin, and by rising from the dead, triumphant over sin and death. These are all truths clearly taught in God's Word.
The reality of man's sinful condition before God is indeed a serious thing. However, as sinful as man can be, the love and grace of God is greater. We are told in Romans 5:20 that "where sin increased, grace abounded all the more." There is no sin that the blood of Jesus Christ shed on the cross cannot cleanse - cp. 1 John 1:7-9 - "if we walk in the Light as He himself is in the Light, we have fellowship with one another, AND THE BLOOD OF JESUS HIS SON CLEANSES US FROM ALL SIN. If we say that we have no sin, we are deceiving ourselves and the truth is not in us. If we confess our sins, He is faithful and righteous to forgive us our sins and to CLEANSE US FROM ALL UNRIGHTEOUSNESS."
It is when a sinner is struck with the goodness of God and His love that he is then awed by the magnitude of the grace of God and trusts in Him. Those who carry the greatest burdens of sin are called upon by Christ to "come to Me, all who are weary and heavy-laden, and I will give you rest." (Matthew 11:28). The forgiveness of sin is referred to in Galatians 5:1 as gaining freedom from the guilt of sin - "It was for freedom that Christ set us free, therefore keep standing firm and do not be subject again to a yoke of slavery [to sin]"
Acts 13:38 declares: "Therefore let it be known to you, brethren, that through Him [Jesus Christ] forgiveness of sins is proclaimed to you, and through Him everyone who believes is freed from all things, from which you could not be freed through [keeping] the law of Moses."
Again, we are told that eternal life (and the forgiveness of sin) is a gift given to us by God received through faith in His Son: "For the wages of sin is death, but the free gift of God is eternal life through Jesus Christ our Lord." (Romans 6:23).
If Jesus Christ cannot forgive all sin, then all men are hopeless and doomed. Through Christ, however, men are given eternal life: "If we receive the testimony of men, the testimony of God is greater; for the testimony of God is this, that He has testified concerning His Son. The one who believes in the Son of God has the testimony in himself; the one who does not believe God has made Him a liar, because he has not believed in the testimony that God has given concerning His Son. And the testimony is this, that God has given us eternal life, and this life is in His son. He who has the Son has the life; he who does not have the Son of God does not have the life." - 1 John 5:9-12.
I trust this helps.