"Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God." (Hebrews 3: 12 KJV)
"He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God...He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him." (John 3: 18, 36 KJV)
"And Jesus answering saith unto them, Have faith in God." (Mark 11: 22 KJV)
"But without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him." (Hebrews 11: 6 KJV)
Are Christians alone saved? Are all non-Christians doomed?
What do these verses teach us? What does Jesus say? Is he being unjust and unkind to limit salvation to only those who believe in him?
Is the sin of unbelief and rejection of Christ that serious? Is it an "unpardonable sin"?
Are the various religions only different ways of finding salvation?
Can't one believe in "God" without believing in Jesus and the "Son of God"?
The answer to these questions is given by the Lord Jesus Christ himself, the one whom Moses foretold would come, "that prophet," who would be also king and priest, the very Son of God, or Lord from Heaven.
Unbelief in Christ is a damning sin. The unbelief that is natural to men is a terrible disease of the soul. It has infected the heart and mind of man. It is natural for him to disbelieve in Christ and the story of his life, passion, and death. Paul said:
"For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God." (I Corinthians 1: 18 KJV)
Unbelief is connected with "hardness" and "impenitence" of "heart," with an "evil heart." An "evil heart" is equated with an "unbelieving heart." A good heart is a heart of faith.
God sent a mighty Savior to rescue men from their sins and their depraved natures and bless them with eternal life and immortality for the body. He has commanded and invited all to come to him for deliverance. It is the greatest insult to his kindness for sinner's to reject his Deliverer, his own Son.
To reject God's Son is to reject him. Jesus said - "he that receiveth me receiveth him that sent me." (Matthew 10 : 40 KJV)
"But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death." (Revelation 21: 8 KJV)
"And brought them out, and said, Sirs, what must I do to be saved? And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house. " (Acts 16: 30, 31 KJV)
(From My Daily Bread blog)