In no particular order
An empty, passive mind can be easily filled with all kinds of wrong thoughts.
A believer who has a passive mind and who does not resist these wrong thoughts often takes them as his own thoughts.
One way to keep wrong thoughts out of your mind is to keep your mind full of right thoughts.
An individual can cast down imaginations repeatedly, but they will always come right back until he learns to fill up the empty space with right thinking.
There are aggressive sins, or sins of commission, and there are passive sins, which are sins of omission. In other words, there are wrong things that we do, and there are right things that we don't do. For example, a relationship can be destroyed by the omission of kind words of appreciation that should have been spoken but never were.
A passive person thinks he is doing nothing wrong because he is doing nothing. Confronted with his error, he will say, "I didn't do anything!" His analysis is correct, but his behavior is not. The problem arose precisely because he did nothing.
H/T: Joyce Meyer: Battlefield of the Mind