There is nothing outside a person that by going into him can defile him,
but the things that come out of a person are what defile him.
That which is coming into you from the outside won't defile (destroy) you, because you let go of it again (verse 18-19). It is only that which you let in and don't let go again, that will destroy you.
Luke 6,45 says, that the good treasures of the heart bring forth good fruits and the bad treasures bring bad fruits, which means that you always need to check what you take in and keep in.
It's really nice to see that principle in your own body. If you eat something then your body makes sure that it keeps that which is essential for LIFE, but at the same time it makes sure that the rest (the bad) is excreted, because if your body would take in all the bad and just collect it, it would eventually destroy you.
Just like your body needs to separate good from bad watchfully, so do you (the Spirit) need to do the same. Every day you are confronted with good and bad, but it's up to you what you let into your heart and collect as treasures.
What do you do when worries, cares, fears, problems, distractions, etc. come against you? Do you collect them in your heart or do you throw them out again?
What do yo do with joy, love, good thoughts, revelations or vision that God gave you? Do you just throw them out or do you make sure to stack them in your heart?
Control yourself and cast out all evil (bad)! Only collect good treasures in your heart, because out of the abundance of your heart your mouth will speak and manifest that which you spoke in your life. (to speak = to act)
So if you are fed up with running from problem to problem and from defeat to defeat, clear out your heart and fill it with good treasures, because only then will you finally bear good fruits!
It’s not what’s passing through you that destroys you, but it’s what you let into your heart and consequently out of your mouth!
You need to always be aware of what you’re filling your heart with, because only the good treasure of your heart (Gods word) can produce good things in your life.
Stack your heart with good treasures, for only then can you be consistent in your words and only then will you see the good fruits! (Mark 7,15-19 & Luke 6,45)