Paul is saying that Christ will not come back until a falling away comes first. That falling away is known as the apostasy. An apostasy is when one leaves their faith to follow someone or something else. So Christ will not come back before the apostasy. In addition, the son of perdition will have to be revealed.
Perdition means loss of the soul, eternal damnation.
Satan is the only one that is sentenced to death at this time. Therefore Satan is the son of perdition. Therefore, we have the apostasy, followed by Satan being revealed for who he is. We still need to know what Satan will look like. Let's keep reading in 2 Thessalonians.