Can a Christian have a Devil (unclean spirit)? Absolutely Yes,  

Why this question is so important
 This is perhaps the single most important question that is asked concerning deliverance. The Bible is filled with scripture on unclean spirits being cast out. Are they called Demons? No, that's not in the Bible, but Unclean spirits is and that's how we refer to them. There are not many churches in today's world that have deliverance ministry, and if you understood the extent of damage they can do to a life or relationship you would be a lot more serious about, dare I say it,, getting advice and knowledge on the subject and looking at yourself. 

One of Satan’s best spread lies, is that Christians cant get unclean spirits in their body (Temple of the Holy spirit). Why?  to keep unclean spirits from being cast out.

If Christians could not have unclean spirits, then what is the point in driving them out, if all we had to do was get the person saved and their unclean spirits would leave on their own? That would leave us casting unclean spirits out of people who refuse to accept Jesus as their lord and saviour! It is my experience that those who claim that Christians cannot have unclean spirits are not even casting them out of non-believers.

If the problem, such as a person having a spirit of infirmity causing disease in their body, is not demonic because they are a Christian and Christians cannot have unclean spirits, then the problem has to be caused by something else, and the real issue that needs to be dealt with is completely overlooked! This is why such a belief among Christians is deadly… literally! Numerous healing that God wants to manifest, do not happen because the spirits behind those particularity infirmities are being completely overlooked.
From www.greatbiblestudy.com

Matthew 10:1
And when he had called unto him his twelve disciples, he gave them power against unclean spirits, to cast them out, and to heal all manner of sickness and all manner of disease.

Matt 12:43-45
"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."

Deliverance is truly not for non-believers. Matthew 12:43-45 is very clear on this, and in truth more harm than good is done. The Bible tells of many times when Jesus Rebuked and cast out unclean spirits from those in torment. Not Demons,,,  but unclean spirits. Jesus did this to those he knew where in the church with belief in him, and those who through the experience of the deliverance gained faith and belief in him, as Jesus knew the beginning from the end, (I and the farther are one). 

Mark 1:23-25
And there was in their synagogue a man with an unclean spirit; and he cried out,
 Saying, Let us alone; what have we to do with thee, thou Jesus of Nazareth? art thou come to destroy us? I know thee who thou art, the Holy One of God.
And Jesus rebuked him, saying, "Hold thy peace, and come out of him."

The right of Access

Truth be known Non-believers should not have deliverance because until a person finds Jesus and a faith and belief in him, the unclean spirits have a right-of-passage to enter into a body when that person opens a doorway. How does a doorway happen? 

Cursing, and repetitive sin like lusting after others or constantly coveting things. using satanic objects like Ouija board, spirit board, playing any games that have a demonic theme.  If Sins are committed they can open a doorway for an unclean spirit to enter into the body and start its works against any chances of salvation.   

 Many Christians believe they have no unclean spirits in them, but sadly this is not true, as its not just about believing in Christ, it's about going through the process of a deliverance ministry and identifying doorways and the related sins and asking for forgiveness of those sins, in order to break the spiritual ties allowing unclean spirits to remain. 

If a Christian has the unclean spirits in them, they are still saved, they don't loose salvation but they do have unnecessary torment and even illness and are more inclined to commit sinful acts or thoughts as they have to fight the subtle evil influence of Satan's unseen workers.

Mark 5:2-9

And when he was come out of the ship, immediately there met him out of the tombs a man with an unclean spirit,
3 Who had his dwelling among the tombs; and no man could bind him, no, not with chains:
4 Because that he had been often bound with fetters and chains, and the chains had been plucked asunder by him, and the fetters broken in pieces: neither could any man tame him.
5 And always, night and day, he was in the mountains, and in the tombs, crying, and cutting himself with stones.
6 But when he saw Jesus afar off, he ran and worshipped him,
7 And cried with a loud voice, and said, What have I to do with thee, Jesus, thou Son of the most high God? I adjure thee by God, that thou torment me not.
8 For he said unto him, "Come out of the man, thou unclean spirit."
9 And he asked him, "What is thy name?" And he answered, saying, My name is Legion: for we are many.  

 Paul said that there was evil indeed present in him:

Romans 7:17-21
Now then it is no more I that do it, but sin that dwelleth in me.
For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.
For the good that I would I do not: but the evil which I would not, that I do.
Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me.
I find then a law, that, when I would do good, evil is present with me.

How did Paul have evil in him, but be filled with the Holy Spirit? Because Paul, like us, was composed of three parts. He was spirit, in a physical body with a soul. A soul is different from the spirit. The spirit is the real you, which is united with the Holy Spirit if you are in Christ. Your soul is your mind, will and emotions, where the darkness can live and remain long after you’ve accepted Jesus and were washed from your sins. This is where unclean spirits can remain, along with your physical body.

The Bible makes a clear distinction between soul and spirit:

 Hebrews 4:12
 For the word of God is quick, and powerful, and sharper than any two edged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.

1 Thessalonians 5:23
And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ.

Your spirit is born again and made new when you accepted Jesus, but your mind, will, and emotions were not born again.

Consider this passage, and it is obvious that all the baggage does not leave a believer’s soul the moment they are born again… but something is made new and old things have passed away – that is because it is speaking of your spirit!

Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.

2 Corinthians 5:17

Was this intended for believers? Or non-believers?
When the even was come, they brought unto him many that were possessed with devils: and he cast out the spirits with his word, and healed all that were sick: That it might be fulfilled which was spoken by Isaiah the prophet, saying, Himself took our infirmities, and bare our sicknesses.

Matthew 8:16-17

Here was Jesus demonstrating the benefits of the new covenant between God and His people, and in doing so, He was driving out demons and healing the sick. Now let me ask you this: Was Christ’s payment for our physical healing (by His stripes we are healed) for non-believers or believers? Believers of course! Then I believe this passage makes it very clear that deliverance (involving casting out demons) and our physical healing is a benefit meant for the believer!

You don’t have to be a lunatic to have unclean spirits!

Many think you have to be a lunatic in order to have unclean spirits, which is not true, and any studying of this subject in the Bible will make this very clear. Consider the man who was perfectly sane mentally, but could not speak nor see because he had an unclean spirit which hindered his speech:

Matthew 12:22
Then was brought unto him one possessed with a devil, blind, and dumb: and he healed him, insomuch that the blind and dumb both spake and saw.

Another example is the woman who was bound with a spirit of infirmity. She was clearly not a lunatic, but merely had a physical infirmity caused by an unclean spirit:

Luke 13:10-16
And he was teaching in one of the synagogues on the sabbath. And, behold, there was a woman which had a spirit of infirmity eighteen years, and was bowed together, and could in no wise lift up herself. And when Jesus saw her, he called her to him, and said unto her, Woman, thou art loosed from thine infirmity. And he laid his hands on her: and immediately she was made straight, and glorified God. And the ruler of the synagogue answered with indignation, because that Jesus had healed on the sabbath day, and said unto the people, There are six days in which men ought to work: in them therefore come and be healed, and not on the sabbath day. The Lord then answered him, and said, Thou hypocrite, doth not each one of you on the sabbath loose his ox or his ass from the stall, and lead him away to watering? And ought not this woman, being a daughter of Abraham, whom Satan hath bound, lo, these eighteen years, be loosed from this bond on the sabbath day?

Nowadays, the word possessed infers ownership, such as our possessions, but  in the NT Greek, we find an entirely different meaning. The phrase possessed with a devil translates back to the Greek word daimonizomai, which means to have a Devil or to be vexed (meaning tormented) by one. Devils (unclean spirits) do not own people, there is no scriptural evidence of such a thing – not even in the life of a non-believer!

If you look up the accounts in the Bible of Devil (unclean spirits) possession, we find people who either have unclean spirits or are being tormented by them. This can manifest in both the physical body or the soul (a person’s mind, will, and emotions).

Many say ''the deliverance ministry ended with Jesus’ ministry" This is not true, and we don’t have to read far into our Bibles to find this out. After Jesus rose again, He appeared to His disciples and gave them the great commission:

Mark 16:15-17
And he said unto them, Go ye into all the world, and preach the gospel to every creature. He that believeth and is baptized shall be saved; but he that believeth not shall be damned. And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues;
"They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover."

If you read the book of Acts, you will find that the early church was also active in the ministry of deliverance, and there are few churches in our times that do this now.