I know how, Hope. There's a verse in one of John's epistles, I think it is, that says — no, wait, here it is in 2 Thess 2: Apparently some people were teaching that the day of the Lord had already happened, and Paul is saying here that "it will not come unless the apostasy come first, and the man of lawlessness be revealed....And you know what restrains him now, so that he will be revealed in his time. For the mystery of lawlessness is already at work; only He who now restrains will do so until He is removed. Then that lawless one will be revealed..."
When I was a teenager I, like most pretribbers here I expect, read The Late, Great Planet Earth by Hal Lindsey, who teaches pretrib. I bought into that teaching at the time, until I started actually reading the Bible seriously for myself. Recently I went back to Lindsey to understand how he arrived at this conclusion, and it's pretty explicit: He reasons that it must be the Holy Spirit who is holding back the antichrist, as Paul says in Thessalonians, and therefore that the Holy Spirit must be withdrawn in order for the antichrist to take over. That's simple and even obvious, and everything else follows from that belief.
(I don't mean Lindsey invented this doctrine—I'm sure it's been around a while. But Lindsey was very clear in expounding it.)
When I started reading the Bible for myself—a lot, not just here and there—I saw that the reasoning must be wrong, because all references to the rapture (and there are a lot of them) link it to the return of Christ in the end. Even the passage in Thessalonians mentioned above does the same thing, starting out "Now we ask you, brothers, regarding the coming of our Lord Jesus Christ and our gathering together to Him, that you not be quickly shaken from your composure or be disturbed either by a spirit, or a message, or a letter as if from us, to the effect that the day of the Lord has come." The part in bold is the point here; the one and the other are linked. To be even more explicit, look in Matthew 24 and Mark 13, both of which describe the "great tribulation" and then say "immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light, and the stars will fall from the sky, and the powers of the heavens will be shaken. And then the sign of the Son of Man will appear in the sky, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of the sky with power and great glory. And He will send forth His angels with a great trumpet blast, and they will gather together His elect from the four winds, from one end of the sky to the other."
Everywhere I see the rapture described, the same thing is going on; it's associated with the return of Christ. So I'm post-trib.
And anyway, as the wife of a church buddy remarked some years ago, when have Christians ever not been in the thick of things?