Why the Bible must get memorized

Stand therefore, having your loins girt about with truth, and having on the [the heart-armor] of righteousness;
…
Above all, taking the shield of faith, wherewith ye shall be able to quench all the fiery darts of the wicked.
Ephesians 6:14, 6:16

We know from Peter that God is telling us to gird the loins of our mind with the truth (pages of memorized Bible):

Wherefore gird up the loins of your mind, be sober, and hope to the end for the grace that is to be brought unto you at the revelation of Jesus Christ;
I Peter 1:13

If a Christian sin, then they drop the shield of faith, lose righteousness, so their heart is exposed to entry by demonic spirits.

If there is Word of God dwelling inside the heart:
… the Word of God is not bound (cannot get bound)
II Timothy 2:9
So demons can’t bind the Word of God, and can’t take over the heart.

But if there is no Word of God dwelling inside the heart:
When the unclean spirit is gone out of a man, he walketh through dry places, seeking rest, and findeth none.
Then he saith, I will return into my house from whence I came out; and when he is come, he findeth it empty, swept, and garnished.
Then goeth he, and taketh with himself seven other spirits more wicked than himself, and they enter in and dwell there: and the last state of that man is worse than the first. Even so shall it be also unto this wicked generation.
Matthew 12:43-45

So memorize the Bible, and make sure there is Word of God dwelling in your heart.