"Ease can put people in danger. Today, people are at ease and no one goes to church." said the preacher during the opening moments of his sermon. Just like the parable Jesus told in Matthew 22. It's the story of the father having a wedding banquet for his son. He sent his servants with invitations for the event. Verse 5 clearly states that the guests made light of it and went on doing other things.
Here comes the danger part of being so casual and comfortable: verse 7. After the servants were mistreated and killed, the father sent his army to deal with the murderers and burn their cities. It's a parable, yes, but it's saying something about God. God calls (invites) people that are at ease (comfortable living in their sin, maybe even thinking that they are good people). Woe to them for making light of it.
A day of reckoning is coming. No one knows exactly when. That's the tricky part. God wants people to be ready, the devil doesn't. It's just like the shepherd tending his flock. He doesn't know when danger (a lion, coyote, or bear) will appear. He is always ready for it, though. Knowing that being vigilant is the best approach to dealing with the enemy will save his sheep.
In Matthew 24, Jesus gives the disciples a glimpse into the end times. Verse 12 also says that many will grow cold and indifferent. On the heels of that is verse 13 which says that those who endure to the end shall be saved. Jesus said it in Luke 9:62, "...no man, having put his hand to the plough, and looking back, is fit for the kingdom of God."
Woe to them that are at ease. A warning to stay in the fight. There's a very real war going on between good and evil, heaven and hell, God and Satan. Satan is a wily character. He would gladly lull people into a sense that it's not really as bad as all that. Retirement for christians is not until Christ returns.