Beware of passionate attachments to the world. Although they deceive you with peace and comfort, they are so fleeting that you do not notice how you are deprived of them, and in their place come sorrow, longing, despondency, and no comfort whatsoever.

St. Leonid of Optina

Whatever the soul may think fit to do itself, whatever care and pains it may take, relying only upon its own power, and thinking to be able to effect a perfect success by itself, without the co-operation of the Spirit, it is greatly mistaken. It is of no use for the heavenly places; it is of no use for the kingdom – that soul, which supposes that it can achieve perfect purity of itself, and by itself alone, without the Spirit. Unless the man who is under the influence of the passions will come to God, denying the world, and will believe with patience and hope to receive a good thing foreign to his own nature, namely the power of the Holy Spirit, and unless the Lord shall drop upon the soul from on high the life of the Godhead, such a man will never experience true life, will never recover from the drunkenness of materialism; the enlightenment of the Spirit will never shine in that benighted soul, or kindle in it a holy daytime; it will never awake out of that deepest sleep of ignorance, and so come to know God of a truth through God’s power and the efficacy of grace.

St. Macarius the Great

Sinful thoughts continually disturb a man. But if he does not cooperate with them, then he is not guilty of them. 

St. Ambrose of Optina

“I have not come,” says the Lord, “to bring peace on earth (that is, love of parents for sons, and love of brothers for brothers who have resolved to serve Me), but war and a sword” in order to separate lovers of God from lovers of the world, the lovers of material things from lovers of spiritual things, the lovers of fame from the humble-minded.

St. John Climacus

We must consider all evil things, even the passions which war against us, to be not our own, but of our enemy the devil. This is very important. You can only conquer a passion when you do not consider it as part of you.

St. Nikon of Optina

Run from places of sin as from the plague. For when fruit is not present, we have no frequent desire to eat it.

St. John Climacus

Why do demons wish to excite in us gluttony, fornication, greed, anger, rancor and other passions? So that the mind, under their weight, should be unable to pray as it ought; for when the passions of our irrational part begin to act, they prevent the mind from acting rationally.

St. Nilus of Sinai

Not every "good" deed is actually good, but only that good deed which is done for the sake of God.  The externals of a particular action are not its essence; God alone sees a person's heart.  We need to humble ourselves, realizing that every good deed is mixed with some passion.

St. Nikon of Optina

We have within us deeply rooted weaknesses, passions, and defects. This can not all be cut out with one sharp motion, but patience, persistence, care and attention. The path leading to perfection is long. Pray to God so that he will strengthen you. Patiently accept your falls and, having stood up, immediately run to God, not remaining in that place where you have fallen. Do not despair if you keep falling into your old sins. Many of them are strong because they have received the force of habit. Only with the passage of time and with fervor will they be conquered. Don’t let anything deprive you of hope.

St. Nectarios of Aegina