You can back up several verses to verse 2 to easily refute their heresy.
1 Thessalonians 5:2
"For yourselves know perfectly that the day of the Lord so cometh as a thief in the night."
Compare this to what Jesus said here:
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Rev 16:14
For they are the spirits of devils, working miracles,
which go forth unto the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty.
Rev 16:15
Behold, I come as a thief. Blessed
is he that watcheth, and keepeth his garments, lest he walk naked, and they see his shame.
Rev 16:16
And he gathered them together into a place called in the Hebrew tongue Armageddon.
As the sixth angel is pouring out his vial of the wrath of God, Jesus has still not "come as a thief" (vs. 15).
Of course, the mark of the beast and his image are already here (vs. 2) prior to Christ's coming as a thief, and so are the seat of the beast and his kingdom (vs. 10).