Satan plays the rôle of a crafty spy; he has access, by some mysterious power, to the heart life of men. At no point of the game for immortal trophies is he so dangerous as when he can take advantage because of his secret knowledge of men's weaknesses and sins. Only a vicious degenerate can be tempted into all the crimes known to the docket of the Bible; few beings on this planet but are fortified at some point of character. They may be weak in many ways--but early training or environment have helped them to become strong in some particulars.
The spy seeks to know when and where a blow may be struck in the enemy's lines, at a place of least resistance. The soul battles are exactly the same; we have no special battles where we are strong; things that might overcome another will mean nothing to us. Our battles are ever fought around the points of weak fortification; the enemy rarely, if ever, has the pleasure of shouting over our downfall from the best that is in us.
The victories of athletic games--the pugilistic bouts in the sporting arena, the mortal duel with rapiers, the battle-fields where thousands fell--have been lost and won by the application of this principle. The general with his field-glass sees a weakening in the enemy's line and orders a charge; the duelist observes a shortening of breath or an awkward movement and seizes the opportunity. It is the weak link in the chain of life that breaks; sins of the lower nature--sensuality--might not appeal to some who fall an easy victim to pride, ambition, or covetousness; others who are liberal, honest to the cent, unassuming, are helpless when tempted in the realm of lower passions. We are at an incalculable disadvantage when our enemy is familiar with our vulnerable points.
So long as the heart is unregenerated and unpurified by the cleansing power of the Holy Ghost, Satan has access to every nook and corner of our heart life. He enters and discovers every vulnerable and invulnerable section of the soul's fortification. The tempted and fallen are often unable to tell how it was done. "Why did you go there?" or, "Why did you do it?" Oh, so many, many times do we hear the answer: "I do not know." A friend once showed me a little iron safe in which he kept his valuable papers. This safe had a very ingenious lock; the combinations were such, and the mechanism so wonderful, that it was capable of _three hundred thousand combinations_.