People might feel they know the reason they have rejected Jesus Christ, but their reason would probably be wrong. The real reason they have rejected the Son of God is stated in the Bible in 2 Corinthians 4:3,4. It says they have been blinded to the truth by an enemy, Satan, who wants them to go to hell and not to heaven.
"But if our gospel be hid, it is hid to them that are lost: in whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, who is the image of God, should shine unto them" (2 Corinthians 4:3,4 KJV).
Now suppose unbelievers continue to walk down a path of darkness and refuse to receive Christ. The end for them does not look good. Unsaved people end up in hell the instant they die. They then know they made a big mistake. They cannot deny the horror and torment they are experiencing. They are tormented not just by flames but by all the opportunities missed by not receiving Jesus Christ as their Lord and Savior. The Lord would have kept them out of the jaws of a fiery hell.
But hell is only a temporary holding place for the damned. After spending time in hell, lost souls will have to eventually stand at the Great White Throne Judgment where they will be judged by the Lord Jesus Christ.
"And I saw a great white throne, and him that sat on it, from whose face the earth and the heaven fled away; and there was found no place for them. And I saw the dead, small and great, stand before God. And death and hell delivered up the dead which were in them. And whosoever was not found written in the book of life was cast into the lake of fire" (Revelation 20:11-12a,13b,15 KJV).
So, what should Christians do? Love the lost. Warn the lost. Intercede, pray for the lost. Win as many souls to Jesus Christ as possible because it is our divine given responsibility. "Therefore all things whatsoever ye would that men should do to you, do ye even so to them" (Matthew 7:12a KJV).