Mark the Ascetic - On the Spiritual Law 167-171

167. The more the intellect withdraws from bodily cares, the more clearly it sees the craftiness of the enemy.
168. A man who is carried away by his thoughts is blinded by them; and while he can see the actual working of sin, he cannot see its causes.
169. It can happen that someone may in appearance be fulfilling a commandment but is in reality serving a passion, and through evil thoughts he destroys the goodness of the action.
170. When you first become involved in something evil, don't say: 'It will not overpower me.' For to the extent that you are involved you have already been overpowered by it.
171. Everything that happens has a small beginning, and grows the more it is nourished.