Luke 17:24-31 and Matthew 24:27 makes it clear that the second return of Jesus will be no secret to anyone. In Matthew, it is especially clear that this statement about the lightning was made so that we wouldn't be deceived by false Christs. Just as lightning is visible to everyone, so the second return of Jesus will be witnessed by the whole world. That's the reason we don't have to be fearful about missing His second coming and anxiously follow every report that Christ has come.

These verses completely destroy the claims of the religions who claim that Jesus has secretly come back the second time.

Just as Jesus had warned His disciples not to be deceived by false Christs because His second coming would be visible to everyone, He also explains that until the very day of His coming, the world will continue on its present course. 

People will not discern the signs of His coming just as the people during Noah's day didn't realize their impending judgment. 

This corresponds exactly with Jesus' prophecies about His coming being like the appearance of a thief in the night. Jesus is emphasizing that the unbelievers will not recognize that He is coming until it is too late. He is stressing that His coming will take the world by surprise.

The Lord is pointing out the urgent need to be ready for His return. In the same way that a thief comes when people are the least prepared, so our Lord will return in a time when people are not looking for Him. 

There will be a condition of apathy in the latter days that will tend to lull even the faithful to sleep if they don't take heed to His words. He urges us to be watchful so we will be prepared.