"Then that lawless one will be revealed whom the Lord will slay with the breath of His mouth and bring to an end by the appearance of His coming; that is, the one whose coming is in accord with the activity of Satan, with all power and signs and false wonders, and with all the deception of wickedness for those who perish, because they did not receive the love of the truth so as to be saved. For this reason God will send upon them a deluding influence so that they will believe what is false, in order that they all may be judged who did not believe the truth, but took pleasure in wickedness." (2nd Thessalonians 2:8-12)
The disciples asked Jesus if many or few would be saved. Jesus answered them, "Strive to enter through the narrow door; for many, I tell you, will seek to enter and will not be able... In that place there will be weeping and gnashing of teeth when you see Abraham and Isaac and Jacob and all the prophets in the kingdom of God, but yourselves being thrown out. And they will come from east and west and from north and south, and will recline at the table in the kingdom of God." (Luke 13:24, 28-29) Later on, He again speaks of those would would be saved and those who would be lost, "For many are called, but few are chosen." (Matthew 22:14) In that day there will be a separation of the people; those who are marked for salvation and eternity in heaven with the Father, and those marked for eternal banishment to hell, far away from the Father's prescience.
Such judgment, the abandonment to eternal darkness and the fires of punishment, can be hard for us to fathom and may even seem to us to be contrary to the loving nature of God. However, such judgment is the righteous reward for those who obstinately oppose God and reject His love and His truth. God has provided a way for all to be reconciled back to Him and to once again be reunited with His love and to enjoy the eternal felicities of heaven in the presence of our Savior, yet for those who reject His truth and His sacrifice for them, only eternal darkness and judgement await them.
For the present, God has allowed both the wicked and the just, the believing and the unbelievers, to live and grow side by side undisturbed, but this won't always be the case. Jesus told us this parable. "The kingdom of heaven may be compared to a man who sowed good seed in his field. But while his men were sleeping, his enemy came and sowed tares among the wheat, and went away. But when the wheat sprouted and bore grain, then the tares became evident also. The slaves of the landowner came and said to him, 'Sir, did you not sow good seed in your field? How then does it have tares?' And he said to them, 'An enemy has done this!' The slaves said to him, 'Do you want us, then, to go and gather them up?' But he said, 'No; for while you are gathering up the tares, you may uproot the wheat with them. Allow both to grow together until the harvest; and in the time of the harvest I will say to the reapers, First gather up the tares and bind them in bundles to burn them up; but gather the wheat into my barn.'" (Matthew 13:24-30) Jesus told us that the wheat are the children of the Kingdom, the tares are the children of perdition, and the reapers are the angles sent at the end of the age to gather all people to their rewards.
Paul, in writing to the Thessalonians is expressing this same truth, telling us that, at the end of the age, God will send the "lawless one" to separate the wheat from the tares, those who believe and those who refuse, that the rewards of each might be made evident and that the righteousness of His rewards and punishments may be made clear and seen by all. In that day none will question God;s justice, for the rightness of His judgments will appear as the difference of heart between the saved and the lost are revealed. For those who have refused the truth that they might be saved will sink further and further into deception and delusion, carried away by the deluding influence of wickedness and pleasures offered by the lawless one. Being offered life, they rather chose licentiousness. Their judgment is just. "You are righteous, O Lord, the One who is and who was and who is to be, because You have judged these things. For they have shed the blood of saints and prophets, and You have given them blood to drink. for it is their just due." (Revelation 16:5-6 NKJV)