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Christians believe that Jesus exists and that He is the one true God but many of them don't believe in Him as their Saviour because they still believe in themselves. They believe that they can save themselves through their own works or through their keeping of the Law. They believe in themselves for salvation and not in Jesus through His finished work. They are guilty of the sin of unbelief through their rejection of Jesus and His sacrifice. To repent means to stop believing in ourselves so we can believe in God and His salvation in Christ because the self believes what is contrary to the word of God.
Many professing Christians will end up in the lake of fire because they rejected the love of God. They died believing in themselves and not in Jesus. They died as unbelievers.
Rev 21:8 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.