It is very possible for demonic spirits to possess and attack a saved Christian. Here are some bible verses you need to know if you are fighting a spiritual attack.
Plead the blood of Jesus
Revelation 12:9,11 KJV
And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.  And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death.
How do you plead the blood of Jesus?
1 John 1:7,9 KJV
But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.  If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
Make it so specific, so vivid to the Lord so that it will become applicable. Any unrighteous thing that you’re going through has to be confessed. This has to be done daily.
Fast and pray
Mark 9:29 KJV
And he said unto them, This kind can come forth by nothing, but by prayer and fasting.
When you fast and pray, your flesh is screaming because all of the darkness is exposed. When you pray, it’s confessed and cleaned up.
Use the full name of the Lord
Luke 10:17 KJV
And the seventy returned again with joy, saying, Lord, even the devils are subject unto us through thy name.
The Father, Son, and Holy Spirit – Lord (Father), Jesus (Son), and Christ (Holy Spirit) – we use this name to ask all of the Godhead to protect us.
Put on the full armor of God
Do not give any space for Satan to attack you. You need a full suit of armor, not partial.
Ephesians 6:11,13 KJV
Put on the whole armour of God, that ye may be able to stand against the wiles of the devil.  Wherefore take unto you the whole armour of God, that ye may be able to withstand in the evil day, and having done all, to stand.
If you don’t have the full armor of God, you cannot stand against the devil.
The full armor of God
Ephesians 6:14-17 KJV
Stand therefore, having your loins girt about with truth, and having on the breastplate of righteousness;  And your feet shod with the preparation of the gospel of peace;  Above all, taking the shield of faith, wherewith ye shall be able to quench all the fiery darts of the wicked.  And take the helmet of salvation, and the sword of the Spirit, which is the word of God:
1) Belt of truth – live a life of honesty and sincerity. The loins are a place of strength.
2) Breastplate of righteousness – we need to be holy and pure. We need holy conduct. That’s why Bible believers separate ourselves from the world from things such as games and entertainment since it creates an opening.
3) Gospel of peace – wherever you go, you should be prepared to witness. That’s what it means to have your “feet shod with the preparation of the gospel of peace”.
4) Shield of faith – blocks the attack of the enemy. When Satan starts to attack, don’t start to think “Why, God?”. You will survive not with discouragement and crying but by knowing that all things will work out for good.
5) Helmet of salvation – once you are saved, you’ve got a foundation laid for blocking demonic attacks. If you’re not saved, Satan will chop your head off and it’s game over.
6) Sword of the spirit – that’s the only offensive weapon you’ve got. Why is dispensationalism, KJV only, and other doctrines important? That’s because all of these doctrines are biblical truths. If you do not have the right doctrine, you have a flimsy weapon. Without a real sword, you will never be able to stab the enemy. That’s why modern churches are dying.