2Th 2:1-4  "Now concerning the coming of our Lord Jesus Christ and our being gathered together to him, we ask you, brothers,  not to be quickly shaken in mind or alarmed, either by a spirit or a spoken word, or a letter seeming to be from us, to the effect that the day of the Lord has come.  Let no one deceive you in any way. For that day will not come, unless the rebellion comes first, and the man of lawlessness is revealed, the son of destruction, who opposes and exalts himself against every so-called god or object of worship, so that he takes his seat in the temple of God, proclaiming himself to be God." 

Now about Jesus returning, don't worry. You won't be left behind. I know that people are saying it has already happened. Others are saying it will happen this day or that! Jesus, Himself, warned of this type of thing happening. So don't get off track, focused on deceptions and speculations. 
That day won't come until the rebellion against the God's temple reveals the lawless one. He will be revealed as one who opposes anything to do with religion and make himself to be as a god. 

You remember I told you about this as has Jesus and John the beloved who was with Jesus. This lawless one only thinks he is god, that he is in control. No, Jesus Christ is in control restraining him until he wants him to be revealed. Though evil, he only does what The Lord allows, as do all who rebel against God and the rule of His Son Jesus Christ.
Yes lawlessness and the mystery of God allowing lawlessness it is already working in the way God had planned from the beginning. Jesus will continue restraining evil until He in His sovereignty decides to clear the way. It is all in God's timing, it is all working according to God's will. 
The lawless are driven by Satan, under His power and control. They will find they are wickedly deceived, it is because they refuse the truth of God's love and reject His call to repentance and salvation. 

2Th 2:5 - 10 "Do you not remember that when I was still with you I told you these things?  And you know what is restraining him now so that he may be revealed in his time. For the mystery of lawlessness is already at work. Only he who now restrains it will do so until he is out of the way. And then the lawless one will be revealed, whom the Lord Jesus will kill with the breath of his mouth and bring to nothing by the appearance of his coming. The coming of the lawless one is by the activity of Satan with all power and false signs and wonders,  and with all wicked deception for those who are perishing, because they refused to love the truth and so be saved." 

So God in the midst of their refusal and rebellion sends them a strong delusion of their own power. They believe what is false and love the sense of not being connected to God. So God is only just in allowing them the condemnation they pursue.

2Th 2:11, 12  "Therefore God sends them a strong delusion, so that they may believe what is false, in order that all may be condemned who did not believe the truth but had pleasure in unrighteousness." 

We are not glad for their destruction, but it should always cause us to rejoice to God that He chose us, gave us the the first fruits of believing so to be saved. This salvation comes from being separated to God and believing the truth of His amazing grace. He is Absolute Love. He called you out through the good news of Christs sacrifice on your behalf and He wants you to share in the Glory of His Son Jesus. 
So hold fast to the truth and what we taught, both in person and by our letters.

2Th 2:13-15  "But we ought always to give thanks to God for you, brothers beloved by the Lord, because God chose you as the firstfruits to be saved, through sanctification by the Spirit and belief in the truth. To this he called you through our gospel, so that you may obtain the glory of our Lord Jesus Christ.  So then, brothers, stand firm and hold to the traditions that you were taught by us, either by our spoken word or by our letter." 

Now lay all that speculation and falsehood aside. Jesus Christ and God the Father loves you. He is your comfort and hope through grace. Don't worry about Him leaving you behind, comfort one another, embolden your hearts for the good work of the proclaiming the word. 

2Th 2:16, 17  "Now may our Lord Jesus Christ himself, and God our Father, who loved us and gave us eternal comfort and good hope through grace,  comfort your hearts and establish them in every good work and word.