Very simply by cross-referencing and comparing scripture.
Since we know from scripture that the wrath of God via the seals, trumpets and bowl judgments must take place in between now and the time that Christ returns to the earth to end that age. And the fact that believers are not appointed to suffer God's coming wrath, then we know that the church must be gathered prior to the first seal being opened.
That is the reason in its simplest form. And the only reason that it will be a secret, as you call it, is because the Lord is going to come like a thief, which means at a time when a person least expects, in secret so that nobody knows he's there, so that he can make off with his goods without anyone knowing it.
This is why I continue to make known that, people don't understand the severity of God's coming wrath, otherwise you wouldn't have people claiming that the church will be going through the time of God's wrath. Then they will tell you, "God is going to protect his church during the time of his wrath. And I say the same thing, they don't understand the severity and magnitude of what is coming upon this earth. It makes more sense to remove the church, then to leave the on the earth and protect them in the midst of it.