I believe the first rapture is only for the faithful, but those not taken are not necessarily non-Christians. I believe the first rapture is a gift, not a right. The Bride will be made up of only the faithful and only God can judge who that is. This is why I heed Jesus' warning and pray:
Luke 21:36 (KJV)
Watch ye therefore, and pray always, that ye may be accounted worthy to escape all these things that shall come to pass, and to stand before the Son of man.
It's just what I think. Not all those left behind are going to hell, they're just not part of the bride. I don't know. It's the only explanation I have for the meaning of this verse. There's a lot of verses that don't fit the end-time scenario, like John the Baptist saying he's not of the bride but a friend of the bridegroom... that implies a ton of stuff that cannot be verified elsewhere in the scriptures - I believe.
As for Luke 21:36, I have no clue. I do know that it is frequently interpreted to mean that we should be closely watching current events so we know how close we are to Christ's return. I have done that myself.
It seems to me that a lot of effort is put into commenting/studying on world events and tying them into biblical prophecy. The underlying assumption is that God wants us to have our finger on the pulse of the news, and this knowledge—along with prayer, will make us worthy to escape all those prophesied things.
However, just a quick study of the Greek word translated "watch" tells me that it has nothing to do with looking at events or keeping world news under close observation.
Contextually then,
We cannot understand verse 36 until we know what preceded it. Verses 34-35 provide the context for Jesus' command to "watch":
"But
take heed to yourselves, lest
your hearts be weighed down with carousing, drunkenness, and cares of this life, and that Day come on you unexpectedly. For it will come as a snare on all those who dwell on the face of the whole earth."
To me, and I may be completely off base here, it seems that Jesus' message is not an admonition to watch world events so that we will know when He will return, but instead, His instruction is to watch
ourselves, which is what "
take heed to yourselves" suggests. He is talking about being vigilant about our own spiritual state, as well as being circumspect and spiritually awake as we go through life.
Speaking only for myself, a subtle danger exists in being too caught up in current events. They can and have caused ME to wind up on some tangent theology. That can distract us from the more vital and important
spiritual preparation that we all need to be under. The bottom line is that the Day will come, you can count on that and we do not know when and will never know when.
Watching events unfold is not what makes us "worthy to escape," but our cooperation with God as He forms His character image in us does. Thus, in addition to prayer, we have to be vigilant in our covenant with Him. We have to "take heed" to ourselves constantly, examining our walk and how we are seeking and imitating God.
Just my thoughts.