That phrase probably brings back memories of a teacher or parent telling you not to do something. The problem was that you were already thinking about it. And you were about to do it! But those words stopped you from doing it. Sadly, people are too often quick to think about what is wrong and not about what is pleasing to God.
In my morning reading, I came across another indication of man's wickedness and complete need for God in regeneration. Psalm 10 is a song about the way wicked people act and think. And in a startling statement, the psalmist very clearly points out their complete disregard for God's ways in verse 4.
The wicked in his proud countenance does not seek God;
God is in none of his thoughts.
How sad to note that without God's intervention, the wicked will not turn from his wicked ways and seek after God. The thought just isn't there. This is why Christians must speak out against sin and point people to Christ. Otherwise, the wicked (a word which encompasses all who have rejected God's ways) will continue in their sinful ways with God "in none of his thoughts."