We don't usually like things sudden.
Like SUD means Sudden Unexpected Death, the connotation is usually not good.
But end-time topics in the Bible have imagery of suddenness.
Here are a few of those imageries to consider...

1. Christ said to learn a parable about a fig tree.
   It suddenly withered.
   
2. The Flood in Noah's time came suddenly, unexpectedly.
   
3. Earthquakes are sudden and unexpected.
   
4. The Bridegroom's coming was sudden for the 10 sleeping virgins.
   
5. The Master's coming was sudden for the slothful servant who wasn't ready.
   
6. Paul wrote that the "day of the Lord" brings sudden destruction.
   

In the face of all this, God says He won't do anything without revealing it.
The next sentence says, "the lion has roared, who will not fear?" Amos 3:7,8.
A lion's roar is sudden also.
And in this case, it's about the Lion of Judah.
"The Lord also shall roar...
and the heavens and earth shall shake.
" Joel 3:16.
So this is about a sudden earthquake.
Different passages use different imagery.
Christ said He would "knock" for the lukewarm church of Laodicea.
A "knock" is sudden, and in this case, the church where He knocked ended in an earthquake circa 63 AD.
The wedding parable in Luke 12:36 also uses "knock," to which we must respond "immediately.
" In most cases, we don't respond to anything immediately unless we have premeditated it.
That's the point-we haven't really thought about the wedding parables, but this one in Luke with a "knock" might be an earthquake, because that's how the lukewarm church (Laodicea) where Christ "knocked," ended.
This information is a "heads up" that we may be surprised by a sudden event more severe that we expect, but in the face of such a calamity, we are to respond with openness "immediately.
"