This verse warns that we who are God's people must not presume, while there is still time for us to get in shape, that, because God has not come down on us like a ton of bricks, everything is fine with our character and attitudes.
Paul includes this verse in a context that lists three or four of Israel's outstanding sins (I Corinthians 10:6-10). Could the ancient Israelites have also thought that they were in good standing when they were not? Were they presuming something? The answer is likely, yes, they showed a careless presumption by their lack of concern about works—by their belief that God is so merciful that He will accept any old attitudes and behaviors and just overlook their sins. However, by doing that, God would not be showing love because the people would not be prepared for the Kingdom of God. Without that preparation, they would not fit into the culture around them and would be absolutely miserable in the Kingdom of God.
The presumption that Paul is talking about is the same flaw that appears in the Laodicean's thinking, revealed when he says, "I am rich, have become wealthy, and have need of nothing" (Revelation 3:17). The members of the Laodicean church felt very good about themselves, but everything was not all right. Their self-satisfaction reveals that their self-evaluation is far from reality, which is that God threatens to vomit them out (verse 16)! We can see what the sin of a Laodicean is. It is presumption, a product of their self-satisfaction that everything is okay.
Because of his lack of faith in the knowledge of God, the Laodicean is deceived into thinking, as Ezekiel 8:12 says, "The LORD does not see us, the LORD has forsaken the land." In other words, he believes that God does not care. But God has always cared—not even for one second since Adam and Eve has He stopped caring!
We must never overlook the principle in Ecclesiastes 8:11: "Because the sentence against an evil work is not executed speedily, therefore the heart of the sons of men is fully set in them to do evil." People presume that everything is okay with the way they are acting. There is a flaw in human nature that persuades men to think that, if God does not immediately punish, He must approve. Yet, do we ever consider that God's non-punishment may very well be the trial that He has imposed on us to see if we will pass it and make the necessary changes ourselves?