Brethren, let us be alarmed at our weakness. Let us be alarmed at sin which so easily deceives us, so easily slinks into us, captures and fetters us. Let us be alarmed at our fallen nature which never ceases to produce the tares of sin. We must constantly watch ourselves, check our conduct and spiritual state with the Gospel, and on no account allow any sinful tendency to grow strong and propagate in our soul by regarding this tendency as unimportant….
We must never neglect tares that spring up from the heart, or sinful thoughts that appear to the mind. Thoughts should at once be rejected and banished, and sinful feelings uprooted and destroyed by opposing them with the commandments of the Gospel and by having recourse to prayer.
Tares are easily got rid of when they are young and frail. But when they take root with time and habit, then their removal calls for the greatest efforts. A sinful thought when accepted and appropriated by the mind enters into the composition of the mind or understanding and deprives it of soundness; while a sinful feeling that lingers in the heart becomes, as it were, a natural property and deprives the heart of spiritual freedom.
~Ignatius Brianchaninov
Prayerful Life 