Our aim is to know God in a direct way, not intellectually or through reason. This is often referred to as receiving the light of Christ. When we receive this light we experience a sense of joy. When it is a light sent by the devil we feel a bit of agitation or obscureness.
Saint Seraphim says,
The Christian heart, when it has received something divine, does not demand anything else in order to convince it that this is precisely from the Lord...
Saint Seraphim also gives many pointers about how to prepare to receive this gift of light.
He says,
To receive and behold in the heart the light of Christ, one must, as far as possible, divert one's attention away from the visible objects. Having purified the soul beforehand by repentance and good deeds, and with faith in the Crucified, having closed the bodily eyes, immerse the mind within the heart, in which place cry out with the invocation of the name of our Lord Jesus Christ; and then, to the measure of one's zeal and warmth of spirit toward the Beloved, a man finds in the invoked name a delight which awakens the desire to seek higher illumination.
What is important to remember that this gift comes after we have purified our heart and soul of its attachment to the passions of the body and one is committed to a life of ongoing repentance and good deeds, including participation in the sacraments of the Church.
Saint Seraphim goes on to describe this gift.
When a man beholds the eternal light interiorly, his mind is pure and has no sensory representations, but, being totally immersed in contemplation of uncreated goodness, he forgets everything sensory and wishes not even to see himself; he desires rather to hide himself in the heart of the earth if only he not be deprived of this true good--God.This is the gift that is available to all who are willing to cooperate with God and undertake the necessary preparation.
Reference: Little Russian Philokalia, Vol 1, pp 46-47

In Orthodox teaching we learn that all of Creation is good. After each step of Creation, God said that everything is "very good." (Gen 1:31) So what is the source of evil?
Here is how St. Paisios explained it to a pilgrim.
Evil exists when we make wrong use of the things of God granted to us for our benefit.
It is not bad for someone to have money, but it is bad to be avaricious. Drugs are not an evil thing, when used to relieve the pain of people who suffer. They are bad when used for a different purpose. A knife is a useful utensil, when we use it to cut bread. However, when it is used to hit someone, it becomes a deadly weapon. In this case, it is not the knife which is evil, but the inner disposition of the murderer.
Therefore, we must use everything in the right way, the natural way, not abuse them and go against nature.
Since we are weak by nature, when we are inclined to give in to a passion, we should try to avoid anything that makes us feel vulnerable. We should also be aware that the reason we avoid the causes of our passions is not because they are evil themselves; but rather, because our ill inner disposition does not permit us to use them correctly.
Since we cannot benefit from them, it is better to avoid them, so they do not harm us. At the same time, we should glorify God for His gifts, and blame ourselves for abusing them and thus provoking the evil.
St. Paisios
For his invisible warfare or conflict with man, especially by means of sinful thoughts and imaginations, the fallen angel relies on the mutual affinity of the sins one with another. This conflict never ceases day or night, but it becomes specially intense and furious when we stand for prayer. Then, according to the expression of the holy Fathers, the devil gathers the most monstrous thoughts from everywhere and pours them on our soul. First he reminds us of all who have wronged or offended us. He tries to present all the insults, wrongs and injuries inflicted on us in the most lurid colors. He points out the necessity for retaliation and resistance to them by demanding justice, common sense, the public good, self-preservation, self-defense. It is obvious that the enemy tries to shake the very foundation of prayer, namely forgiveness and meekness, so that the building erected on this foundation may collapse of its own accord. And this is just what happens, because a person who is full of resentment and who does not forgive his neighbor’s sins is quite unable to obtain compunction or concentrate when he prays. Angry thoughts dissipate prayer; they blow it aside, just as a violent wind scatters seeds thrown by a sower on his field; so the field of the heart remains unsown, and all the ascetic’s hard work comes to nothing.
It is a well-known fact that forgiveness of wrongs and offenses, changing condemnation of our neighbors into kindness and mercy so that we excuse them and blame ourselves, provides the only solid basis for successful prayer. …We must follow our Lord’s example and teaching; we must reject thoughts of earthly glory, success, and earthly plenty; we must refuse joy brought by those fancies and reflections which destroy in us contrition of spirit, concentration and attention during prayer, and which lead to self-opinion and distraction. If we consent to thoughts of resentment and condemnation, thoughts and fantasies of vainglory, pride, love of money and love of the world, and if we do not reject them, but dawdle in them and take pleasure in them, then we enter into fellowship with Satan, and the power of God which protects us will leave us. The enemy then rushes at us with two most grievous conflicts: the conflict with thoughts and fantasies, and the conflict with despondency or sloth. Defeated in the first conflict and deprived of God’s protection, we cannot stand against the second conflict either. This means, the Fathers tell us, that God allows Satan to defeat us until we humble ourselves…
St. Ignatius Brianchaninov: “The Arena”
Fear evil like fire. Don’t let it touch your heart even if it seems just or righteous. No matter what the circumstances, don’t let it come into you. Evil is always evil. Sometimes evil presents itself as an endeavor to God’s glory, or as something with good intentions towards your neighbor. Even in these cases, don’t trust this feeling. It’s a wrong labor and is not filled with wisdom. Instead, work on chasing evil from yourself. Evil, however innocent it looks, offends God’s long-suffering love, which is His foremost glory. Judas betrayed his Lord for 30 silver pieces under the guise of helping the poor. Keep in mind that the enemy continuously seeks your death and attacks more fiercely when you’re not alert. His evil is endless. Don’t let self-esteem and the love of material goods win you over.
When you feel anger against someone, believe with your whole heart that it’s a result of the devil’s work in your heart. Try to hate him and his deeds and it will leave you. Don’t admit it as a part of yourself and don’t justify it. I know this from experience. The devil hides himself behind our souls and we blindly think we’re acting by ourselves. Then we defend the devil’s work as something that is a part of us. Sometimes we think that anger is a fair reaction to something bad. But the idea that a passion could ever be fair is a total and deadly lie. When someone is angry at you, remember that this evil feeling is not him. He’s just fooled by the devil and is a suffering instrument in his hand. Pray that the enemy leaves him and that God opens his spiritual eyes, which have been darkened by the evil spirit. Pray to God for all people enslaved by passions because the enemy is acting in their hearts. Perhaps you hate your neighbor, despise him, don’t want to talk to him peacefully and lovingly because he has been rude, arrogant, or disgusting in his speech or manners. You may despise him for being full of himself or proud or disrespectful. But you are to blame more than he is. “Physician, heal yourself!” (Luke 4:23). So, teacher, teach yourself. This kind of anger is worse than any other evil. How could evil be chased out by another evil? How can you take a needle from the eye of another person while having a log in your own? Evil defects must be fixed with love, kindness, resignation, and patience. Admit yourself as the worst of all sinners, and believe it. Consider yourself the worst one, chase away any boldness, anger, impatience and fury. Then you may start helping others. Be indulgent about defects of others, because if you see their faults all the time, there will be continuous enmity. “The plowers plowed upon my back: they made long their furrows.”(Psalm 129:3). ” For if you forgive men their trespasses, your heavenly Father will also forgive you.”(Matthew 6:14). We can feel from time to time the most perfect love for God without loving each other. This is a strange thing, and only few care about it. But love for our neighbor will never come without our own effort.
A real Christian doesn’t have any reason to be angry about anybody. Anger is the devil’s deed. A Christian should have only love inside and since love doesn’t boast, he shouldn’t boast or have any bad thoughts towards others. For example, I must not think about another person that he is evil, proud etc; and I must not think that if I forgive his offense he would laugh at me or upset me again. We must not let evil hide in us under any pretense. Evil and anger usually have many different veils.
Don’t yield to gloomy feelings in your heart but control and eradicate them with the power of faith and the light of the sane mind. These strengths will make you feel secure. Let me not be put to shame, for I take refuge in you. (Psalm 25:20). Gloomy feelings usually develop deep in the heart. Someone who didn’t learn how to control them will be gloomy, pensive most of the time and it will be hard for him to deal with himself and other people. When he comes close to you, sustain yourself with inner strength, happiness and innocent jokes: and they will leave you soon. This is from experience.
Lord, give me strength to love everyone like myself and never to get angry or work the for devil. Give me strength to crucify my self-esteem, my pride, my greed, my skepticism and other passions. Let us have a name: a mutual love. Let us not worry about anything. Be the only God of our hearts, and let us desire nothing except You. Let us live always in unifying love and let us hate anything that separates us from each other and from love. So be it! So be it!
If God showed Himself to us and lives inside us as we in Him (according to His eternal Word), wouldn’t He give us everything? Would He ever , would He ever trick us or leave us? He who did not spare his own Son, but gave him up for us all—how will he not also, along with him, graciously give us all things? (Romans 8:32). Now be comforted, my dear, and know nothing but love. This is my command: Love each other (John 15:17).
St. John of Kronstadt
http://agapienxristou.blogspot.ca/2013/09/evil-is-always-evil.html
[The Muslims] call us Hetaeriasts, or Associators, because, they say, we introduce an associate with God by declaring Christ to the Son of God and God. We say to them in rejoinder: ‘The Prophets and the Scriptures have delivered this to us, and you, as you persistently maintain, accept the Prophets. So, if we wrongly declare Christ to be the Son of God, it is they who taught this and handed it on to us.’
But some of them say that it is by misinterpretation that we have represented the Prophets as saying such things, while others say that the Hebrews hated us and deceived us by writing in the name of the Prophets so that we might be lost. And again we say to them: ‘As long as you say that Christ is the Word of God and Spirit, why do you accuse us of being Hetaeriasts? For the word, and the spirit, is inseparable from that in which it naturally has existence. Therefore, if the Word of God is in God, then it is obvious that He is God. If, however, He is outside of God, then, according to you, God is without word and without spirit. Consequently, by avoiding the introduction of an associate with God you have mutilated Him. It would be far better for you to say that He has an associate than to mutilate Him, as if you were dealing with a stone or a piece of wood or some other inanimate object. Thus, you speak untruly when you call us Hetaeriasts; we retort by calling you Mutilators of God.
* This excerpt is from “Saint John of Damascus: Writings, The Fathers of the Church, Vol. 37″ translated by Frederic H. Chase