I've read that the light shines in that darkness, and that the darkness does not comprehend it. Does this mean that there is more light in a Christian then darkness or can the darkness at times dominate?
In a Christian is there always a little darkness with alot of light? Does that light fluctuate depending on certain things?
The light is God's Word. The darkness is the secular world. Merely believing in God does not bring transformation, but when we act on that belief we start to grow spiritually and the light becomes more real to us than the darkness.
Whatever we take in through our gates (eyes, ears primarily) has an influence in our heart/mind. If we are not reading God's word and living it out...we deceive our own selves..even if we think ourselves a good person...what we perceive as light...is actually darkness if we are not being led by the Word.
'“The lamp of the body is the eye. If therefore your eye is good, your whole body will be full of light. But if your eye is bad, your whole body will be full of darkness. If therefore the light that is in you is darkness, how great
is that darkness!" Matthew 6:22-23
So the light does fluctuate according to our faith and obedience because our attention generally determines our beliefs and realities.
To answer your question whether or not believers can have evil spirits...the short answer is yes. It's not uncommon for a Christian who's living in sinful addictions to have more than one unclean spirit. This is why faith without works is dead.
'What
does it profit, my brethren, if someone says he has faith but does not have works? Can faith save him? If a brother or sister is naked and destitute of daily food, and one of you says to them, “Depart in peace, be warmed and filled,” but you do not give them the things which are needed for the body, what
does it profit? Thus also faith by itself, if it does not have works, is dead.
But someone will say, “You have faith, and I have works.” Show me your faith without your works, and I will show you my faith by myworks. You believe that there is one God. You do well. Even the demons believe—and tremble! But do you want to know, O foolish man, that faith without works is dead? Was not Abraham our father justified by works when he offered Isaac his son on the altar? Do you see that faith was working together with his works, and by works faith was made perfect? And the Scripture was fulfilled which says, “Abraham believed God, and it was accounted to him for righteousness.” And he was called the friend of God. You see then that a man is justified by works, and not by faith only' James 2:14-22
Furthermore, allow me to clarify that evil spirits can only reside in the born-again believer's flesh/body/soul but not their spirit. A clean cut example would be drug addictions. So when a believer slips into serious sin addictions, they may be oppressed or even manifest demons.
A nonbeliever (someone who rejects God) can always be oppressed by demons because they are by nature, children of the devil (children of wrath). Thier entire nature is fallen until they are born again as a new creation.
'And you
He made alive, who were dead in trespasses and sins, in which you once walked according to the course of this world, according to the prince of the power of the air, the spirit who now works in the sons of disobedience, among whom also we all once conducted ourselves in the lusts of our flesh, fulfilling the desires of the flesh and of the mind, and were by nature children of wrath, just as the others' Ephesians 2:1-3
It is for this reason all believers should be baptized in full immersion water (1 Peter 3:21) and begotten of the Holy Spirit and live in ongoing repentance, otherwise they are not being faithful to Christ and give their legal rights over to the devil...who knows what kind of spiritual oppression they may endure in their time of disobedience. The good news is that when we are obedient and filled with the Word and the Holy Spirit (and love) we are mighty warriors for the Kingdom of God and we can destroy the works of the devil.
'But in a great house there are not only vessels of gold and silver, but also of wood and clay, some for honor and some for dishonor. Therefore if anyone cleanses himself from the latter, he will be a vessel for honor, sanctified and useful for the Master, prepared for every good work. Flee also youthful lusts; but pursue righteousness, faith, love, peace with those who call on the Lord out of a pure heart. But avoid foolish and ignorant disputes, knowing that they generate strife. And a servant of the Lord must not quarrel but be gentle to all, able to teach, patient, in humility correcting those who are in opposition, if God perhaps will grant them repentance, so that they may know the truth, and
that they may come to their senses
and escape the snare of the devil, having been taken captive by him to
do his will' 2 Timothy 2:20-26 (NKJV)