"And then the lawless one will be revealed, whom the Lord Jesus will overthrow with the breath of his mouth and destroy by the splendor of his coming." NIV translation
Paul reminds us that even when the beast is revealed, it will only be for a short time. He reminds us that Jesus will defeat him when he appears again. This fake lord will fade quickly when the real Lord shows up.
"The coming of the lawless one will be in accordance with the work of Satan displayed in all kinds of counterfeit miracles, signs and wonders, and in every sort of evil that deceives those who are perishing. They perish because they refused to love the truth and so be saved." NIV translation
This beast will be able to perform all kinds of miracles and signs. Many will be deceived and follow the beast because they abandon the Truth which is Jesus Christ. If we love Jesus, we will want to know more about Him and a big part of that is studying His word. If we know His word, we will not be deceived by the schemes of the devil or his demons.
"For this reason God sends them a powerful delusion so that they will believe the lie and so that all will be condemned who have not believed the truth but have delighted in wickedness." NIV translation
This delusion is when the beast has a fatal wound but does not die (see Revelation 13:3 & 4). Those that do not trust in Jesus and have the Holy Spirit will believe this lie and worship the beast. They did not believe the truth that Jesus is the Son of God and so did not receive the Holy Spirit.
"But we ought always to thank God for you, brothers loved by the Lord, because from the beginning God chose you to be saved through the sanctifying work of the Spirit and through belief in the truth." NIV translation
If you are in Christ, it is because God chose you from the beginning of time and called you through the working of the Holy Spirit. Nobody comes to God on their own; we all come as we are called by the Spirit of God. When confronted with the truth of His Word, we accept it and believe because of the work of the Spirit.