How can we be sure that the Rapture is before the Tribulation?

The answer to this question is not an easy thing to define.  It is helpful to know where the word "Rapture" comes from.  Rapture means "caught up" and is not found in the English Bible, but is found in the Latin Bible.

So what evidence in Scripture is there that the Rapture occurs before the Tribulation?  Again, not an easy thing to define, but the clearest Scripture is found in 1 Thessalonians 5 v 9, "For God hath not appointed us to wrath, but to obtain salvation by our Lord Jesus Christ."  The Tribulation period is when God pours out His wrath in righteous judgement on the Earth.  At the end of Chapter 4 Paul outlines the comfort that we can give to each other at the time of departure, when a believer passes on.  In chapter 5, Paul seeks to describe the events surrounding the Rapture, including that we will not face the wrath of God.  How do we know that the Tribulation is the time when God will pour out His wrath?  Revelation 6v16 says, "And said to the mountains and rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb, For the great day of his wrath is come; and who shall be able to stand?"  Those who are seeking to hide from this wrath are those left behind.  Note it is the wrath of the Lamb, Him who evil and cruel men took and crucified will pour out the wrath.  Note that the Father is portrayed as "him that sitteth on the throne", while the Son is portrayed as "the Lamb".  John the Baptist could point and say, "Behold the Lamb of God which taketh away the sin of the world."