For healthy, joyful, peaceful living it is vital to keep your door closed to evil.
For continuing spiritual growth it is essential to keep your door closed to evil.

Overall, this world is not as positive for healthy living and smooth spiritual progress
as it could be: plenty of negative energies are mixed in the pot.   If you would like to live
healthy, joyful, peaceful, spiritual life, not giving these negative energies an opportunity
to enter your life is vital; thus masters have been saying: when it comes to evil,
do not go there.

When you are healthy, your intelligence naturally guides you to stay away from evil.
When your health is compromised, your intelligence can get suppressed and you may not
be able to tell "good" from "bad". If you open your door to the "bad" (evil), it (the evil) will
try to attract other negative stuff to you, like a magnet.   The more negative stuff enters your
door, the more your intelligence, your internal guidance, gets clouded.   If not stopped
and healed, this can snowball into a big mess.   It can be seen in people who do something
stupid, common-sense defying, or even criminal.   Upon hearing about their mishaps or
legal problems you may just roll your eyes and sigh, "don't these people have any common
sense?".   No, they don't.   They get so scrambled by whatever negative energy affects them
that they can't think straight.
If you allow your life to be open to something negative, even just a little bit, the negative
will enter.   Once something negative is a part of your life, it will exploit you from
the inside as best and as much as it can, including attracting other negative stuff to you.
Some evil seems obvious, such as murdering someone or taking illegal drugs.   There are
many other forms of evil that seem "innocent", but their entertainment can end up even
in murder and drug addiction.   Some examples: violent movies; horror books; social
alcohol drinking; extreme sports; violent computer/video games; eating junk food;
smoking; porn; violent thoughts; etc.

For better clarity here are some examples:

1) a boy plays a computer game; let's say that to play this game frogs are shot from
a slingshot and then exploded mid-air to take out targets and earn points.   Regardless of
how cute and funny this game may look graphically, it uses the concept of violence for
entertainment. Concepts and thoughts have real energy.   The energy of violence is
a negative energy (evil), of course.   Thus, the boy has opened his door to evil.
(In truth, just the fact that this boy was willing to play this computer game in the first
place indicates an incongruence [a disharmony] in his energy field, in his life.
Thus, actually, evil entered through his door even before that - perhaps via
junk food eating.)   Every time evil enters someone, intelligence is affected.
So now, after playing this computer game, the boy has somewhat less positive internal
guidance.   In addition to that, the negative energy that entered him tries to bring him
down and attract other negative stuff to him.   Now the boy goes to school and sees
a group of kids bullying another kid. Normally he would avoid such stuff, or perhaps
even try to break it up, but this time the negative energy inside of him has clouded his
reasoning so he joins in the bullying.   His door has opened even wider to evil.
More negative energy in his life = a stronger magnet to attract other negative energies
to him.   The next time he comes across alcohol, his intelligence may be sufficiently
impaired for him to give the drink a try.   Alcohol is one of the worst negative
energies in existence - who knows how far this can snowball...

2) she likes to entertain erotic thoughts.   There is nothing wrong with that per se.
One day a negative energy comes by and knocks on her door.   To her conscious mind
this manifests as an erotic thought to be tied to a bed and to have her body touched
a little bit rougher.   She thinks, "hey, that's not too bad and it sounds so exciting."
So instead of ignoring the thought she opens her door to it and entertains it.
Now the negative energy gained a tiny foothold in her life.   This means somewhat
less intelligence and more vulnerability to evil.   As the result, when the next negative
temptation in the form of even a more violent erotic thought comes in, she has
less resistance to ignore it and more inclination "to go there", to entertain it.   Little by
little these violent erotic thoughts escalate.   With each one her door is more open to evil,
and not just in the domain of erotic thoughts, but all across the board.   If this is not
stopped and healed, she may go mad, or drown in negative emotions,
or be affected by poor health, or get into an accident, etc.

It is unlikely for a healthy individual to be all of a sudden affected by a big negative
energy and to do something stupid or even criminal right away.   It is much more likely,
almost certain, it starts with a little bit of disharmonious here and a little bit of
disharmonious there.   Like cracks in a dam. A few cracks may affect the structure
to some degree, but not obviously so.   However, once there are many cracks,
the dam is likely to fail, and then a disaster strikes.
As you go through your life and interact with all the stuff around you, you consciously
don't know what the state of your being is and what you can afford to handle safely and
what not. Consciously you may not be aware of the smaller cracks.   By the time
the accumulated cracks manifest in your life as something unpleasant, it may be a big
mess inside of you.   That is why it is critically important to be aware of the thoughts,
events, feelings, things, and people you come into contact with, and ignore those that
are not harmonious with Love.   A good guidance can be: "would this be what
Christ/Buddha/ultimate love/the highest good would do/use/eat/drink**/entertain?"
If the honest answer is "no", it is the best to ignore it.   In this world it is impossible not
to be approached by evil, but it is possible to ignore it.   For that you need your health
to be at the highest level; the page Good Health Trinity provides more answers about that.
Neuro-Emotional Technique (NET) can reveal the true face of things and it can help
reveal and heal negative affections (cracks) in your life.

Please remember, what you think you can clearly see as negative now may not be so
if you get affected by negative energies.   Negative energies cloud, suppress intelligence.
The evidence of this is aplenty in this world.   How else could there be so much struggle,
violence, and stupidity. That's why the masters say: when it comes to evil, do not go there;
do not even consider it as a joke.

**Note: the Bible says that Jesus turned water into wine.   This is nonsense, of course.
             An individual as highly evolved as Christ is aware of the negativity and danger
             of alcohol and wouldn't support it in any way.