"He will oppose and will exalt himself over everything that is called God or is worshiped, so that he sets himself up in God's temple, proclaiming himself to be God." NIV translation
This beast will set himself up and try to replace God. Satan wanted to replace God and that was why he was thrown out of heaven along with one third of the angels. This beast will mock and oppose everything that has to do with God and those that accept Jesus will be killed. We may ask ourselves how this could be but, if we look around us today, it seems that many have already set themselves up in God's place.
"Don't you remember that when I was with you I used to tell you these things?" NIV translation
Paul is reminding the Thessalonians that he is not telling them anything new. God does not want us to be ignorant and so He provides teachers and His word to help us to grow. If we are growing and grounded in the word, then we will not be easily confused by false prophets and false reports.
"And now you know what is holding him back, so that he may be revealed at the proper time." NIV translation
Paul reminds them that it is God who is in control and He is keeping this beast from taking control of the earth. There is an appointed time for this but only the Father knows when it will happen. Sometimes, we want to ask why God doesn't just take the church out of the world and get on with it and the answer is it is not the appointed time. God is merciful and He is waiting for all of His people to accept His grace. But, the appointed time could be today or tomorrow so we must be prepared.
"For the secret power of lawlessness is already at work; but the one who now holds it back will continue to do so till he is taken out of the way." NIV translation
Satan is at work in the world today as we can easily see by looking around us. But, as bad as things appear today, they are going to be much worse when this man of lawlessness is able to reveal himself (see Revelation 13). Most of the time this power is at work behind the scenes because the anti-christ has not been allowed to reveal himself. This verse reminds us that even in this matter God is in control and the devil and his workers cannot just do anything they want.