The devil cannot attack a chriatian from outside effectively thus demons are always seeking opportunities to gain entrance within so as to operate from inside. Little wonder the bible says "guide your heart with all deligence for out it are the issues of life"
Ephesians 6:11-12 NLT
"Put on all of God’s armor so that you will be able to stand firm against all strategies of the devil. For we are not fighting against flesh-and-blood enemies, but against evil rulers and authorities of the unseen world, against mighty powers in this dark world, and against evil spirits in the heavenly places"

Glad you finally recognize the basic problem of christianity, even though i disagree that the devil is at the center of it. You see, christianity, in it's central doctrine, completely ignores the inner spiritual workings while concentrating fully on the outer. Like outer rituals, books, and outer stuffs like the silly obsession with the status of their figure of worship and their worldly labels or IDs as "christians". They are so obsessed and fixated on all these external things that they have been made to forget the internal state of being which is far more important. It is this fixation on the outer things that have made this religion to be one of such shallowness.

Jesus was about the inner journey, while christianity, or more accurately, churchianity(which most of you are part of these days) is about the external journey that ultimately leads to nowhere.