Few New Testament passages are as difficult as this one! Augustine, founding father of much of Western theology wrote 'I frankly confess I do not know what it means'!
The passage echoes what Paul wrote before about Jesus' return. Perhaps a misunderstanding had crept in, hence this second letter. The problem for us is that Paul also refers to verbal teachings that we simply no longer have.
Yet Paul is making clear that until Jesus does return evil will loom large, and if anything will increase. He talks of a Great Opposer leading people away from Christ, setting itself up as god. Yet interestingly somehow also held back by God, at least for a time.
Perhaps God's mechansism for this is the law and order normally found in stable society. If we look at places in the world today where such law and order has disintegrated, we certainly see people or organisations setting themselves up claiming absolute allegiance and supremacy. Yet such activities are not new - Roman emperor Gaius tried to desecrate the Jewish temple in AD40.
It is how we react as followers of Christ that is important. Rather than being consumed with worry or fear, concentrating on the bad, we should look to Christ. Our calling in the face of whatever evil is to Stand Firm (v15). We can do this because of the death and resurrection of Jesus. Its not that we simply bury our heads and hope for a fairytale ending, but because we know that Jesus already has the victory assured.
Some seem to want all the future detail spelled out in order to find confidence, yet maybe we should simply be content with a child-like faith and simply know and trust God for the outcome. The big picture rests in His hands. We need not worry or be fearful, for the breath of God can remove any and all opposition at the time of God's choosing. He will provide a way for us even in troubled times, His perfect love holding all those who look to Him.