The mind is a canvas. And it takes discernment (and patient practice) to know who is painting on it… us, or the culture or the enemy or God. Not all thoughts we have are from us (and many of our own thoughts are…uh..not that helpful). We cannot afford to be passive as we meander through our brains. We have to be aware and disciplined, and to know the difference between the stuff in our heads and the Word of God:
“…we take captive every thought to make it obedient to Christ.”
(2 Corinthians 10:5)