So many times, we go through life and a thought comes to our mind.
Yet we let it pass and don’t think a thing about it later on. We tend to think, if I do not say that this thing/thought can affect me, then it can’t. Here is the problem

a thought unchallenged is like giving it approval to stay.
What do I mean? Well, if we do not challenge a thought, do not correct the lie, it is as if we have accepted it and made a room for it to stay.
It is kind of like a bad piece of fruit, you bring it in unintentionally and when you notice it you acknowledge its presence, but you have to make a choice. On one hand you can deal with the spoiled fruit and get it over with, making sure that it does not affect the other fruit. Yet on the other hand you could allow yourself to put it off for a time.

When this happens many times we forget about it and it just sits there and festers. It grows and begins to spread. As it spreads it starts to rot all that is around it, polluting the whole container of fruit. This is just how our thoughts are inside the mind. It must be dealt with up front and made ineffective to not give the opportunity for it to grow and manifest into something more.
The word of God tells us that we are fighting more than just what we can see and feel through the natural senses.

2 Corinthians 10:4-5 KJV
4(For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds;) 5 Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ;

Now you may say that this is out of context, that Paul is talking about taking captive any intellectual argument or idea that is setup against Christ. Yet we cannot discount that the thoughts inside out head are intellectual arguments or ideas, either these thoughts we deal with are introduced from society or demonic influence. Some may say, what demonic influence? Well the very thoughts that we face every day could easily be an attack of the enemy. The use of the term, “every fiery dart” depicts the image of a flaming object being hurled at you.

Ephesians 6:16 ESV
16 In all circumstances take up the shield of faith, with which you can extinguish all the flaming darts of the evil one;
In its common form this dart is most commonly an arrow or other projectile that is ether dipped in tar, pitch or some other flammable material and on impact transfers the flame as an incendiary attach against the opponents.
Even if it was not a direct hit, maybe seeming unimportant, if not dealt with, the flame would spread and eventually overtake the recipient.
So it is critical for the attack to be dealt with immediately. One must take a conscious effort, even when it is not convenient. This way the interjected thought that has the power to take root and take over is effectively made null and void.
When we come against the thought the moment that it enters our mind gate, we take captive its intentions, canceling its effective task, realigning our thinking intentionally into the ways and teachings of Jesus.