If we read the top part of the verse, it says if we Sin willfully........... that is one sin.
the part you posted is what the sin was and is related to Heb 6. That is leaving Jesus, and going back to judaism. counting the covenant were we were santified a unworthy thing.
So, those that are left behind may not qualify for that verse. The scripture says if we sin willfully. That meaning we knew exactly what sin we were commiting.
If we read Heb 6 we find that a person must qualify for this sin. Most do not, and millions of christians have not even experienced the power and Gifts of the Holy Spirit. So they Would have no concept of what they were leaving for another god, or type of God.
This sin in Hebrews, The sin of rally against the Holy Spirit, and the sin John talks about, fall into this catagory when John said there is a sin unto death, and not to pray for that, He then warned at the end to stay away from idols. Leaving God, and the Precious Holy Spirit being counted as worthless.
As for "Left Behind" Jesus comes for His Church. This is without the pressures of the End time tribulation but just the everyday life we go though. Not doing as the Lord called you to do, and being on the Path you should be on, means that Jesus is not really your Lord, and you have chosen to do your own things.
Mat 24:44 Therefore be ye also ready: for in such an hour as ye think not the Son of man cometh.
Mat 24:45 Who then is a faithful and wise servant, whom his lord hath made ruler over his household, to give them meat in due season? Mat 24:46 Blessed is that servant, whom his lord when he cometh shall find so doing.
Mat 24:48 But and if that evil servant shall say in his heart, My lord delayeth his coming;
Mat 24:49 And shall begin to smite his fellowservants, and to eat and drink with the drunken;
Mat 24:50 The lord of that servant shall come in a day when he looketh not for him, and in an hour that he is not aware of,
Mat 24:51 And shall cut him asunder, and appoint him his portion with the hypocrites: there shall be weeping and gnashing of teeth.
Luk 12:46 The lord of that servant will come in a day when he looketh not for him, and at an hour when he is not aware, and will cut him in sunder, and will appoint him his portion with the unbelievers.
Luk 12:47 And that servant, which knew his lord's will, and prepared not himself, neither did according to his will, shall be beaten with many stripes
Here we see a case where Jesus comes and finds His servants being faithful to what they are suppose to be doing, or doing their own thing.
Many just do as they wish during the Week, but are in church every Sunday. Jesus is not really Lord to them, yet they call themselves Christians. Even when getting money, they don't put the Lord's things first, but consider what they need to pay or want.
They also do not take heed about any call of God that is on thier life, though it is not hard to find.
Jesus said that he will appoint them his portion with the unbelievers when he comes. That means "Left Behind" as they are not part of the Church, nor do things that further the Gospel, or do it with a half a heart. Jesus said in both statements that, they do not know when He is going to show up.
Mat 24:36 But of that day and hour knoweth no man, no, not the angels of heaven, but my Father only.
Mat 24:37 But as the days of Noe were, so shall also the coming of the Son of man be.
Mat 24:38 For as in the days that were before the flood they were eating and drinking, marrying and giving in marriage, until the day that Noe entered into the ark,
Mat 24:39 And knew not until the flood came, and took them all away; so shall also the coming of the Son of man be.
Mat 24:40 Then shall two be in the field; the one shall be taken, and the other left.
Mat 24:41 Two women shall be grinding at the mill; the one shall be taken, and the other left.
In this event we see the same thing, jesus said we will not know when he comes. This is also the event such as Noah that we escape destruction, but instead of going by boat, He said we are taken, the ones that He finds faithful. Those Christians not putting Christ and the Church first, will be left with the rest of the unbelievers, but unlike Hebrews chapter 6 and 10, There is still hope for them.
Jesus Is Lord.