Thomas A Kempis 1.13 (part 1) - The root of temptation lies within us

So long as we live in the world, being without trouble and temptation is impossible. So it is written in Job, Troubled is the life of man on earth. So each of us should be concerned about his own temptations and be vigilant in prayer, so that the devil finds no place deceive; who never sleeps, but circles around seeking his prey. No one is so holy and perfect that he never has temptations, and to be fully free of them is not possible. 

However, there are temptations that are very useful, even though they afflict us seriously; for through them we are humbled, purified, and instructed. All the saints passed and progressed through many trials and temptations. Those who did not endure were held back by them and failed. There is no station in life so sacred, no place so secret, that there are no temptations and adversities.

There is no man completely free from temptations, no matter how long he has lived, because the cause of temptation is within us. We are born with disordered desires. As soon as one trial or temptation fades, another takes its place, and there is always something that we need to endure patiently. This is because we have lost the good of happiness. Many seek to flee from temptations and fall more seriously into them. We cannot win the fight by running away, but by patience and true humility we become stronger than all our enemies.

The one who resists the outward part of the temptations but does not pull up the root, that one will find that temptations return more quickly and rage more fiercely. By small steps, through patience and endurance, with God's help you will conquer better than with your own hardness and self-willed persistence. Often accept council and consolation in temptation, and with someone who is tempted do not act harshly, but comfort and strengthen him as you would have done to yourself. 

Imitation of Christ by Thomas A Kempis, 1.13 (first part, it's a long chapter to translate in one sitting). 

Translation focused on contemporary English and preservation of rhetorical force and art