The Ramchal said: "If the knowledge of G-d was clear to us, we wouldn't sin." Every sin therefore comes from a lack of belief in HaShem. Someone who has 100% total emunah will never come to sin.
The Midrash relates to us that when Moshe Rabeinu was growing up in Pharoah's house he one day, as a child, removed Pharoah's crown from his head. His advisors told him that this was a sign that Moshe was to be the savior of the Jews. However, others claimed that since Moshe was still an infant he was too young to be responsible for his actions. So they put him to the test: they brought him two bowls. One of gold and silver and one of hot coals. Moshe wanted to take the gold and silver but an angel came and pushed his hand over to the coals . . . by way of "drash" one could explain the Midrash as follows: The evil inclination was telling Moshe to take the gold and silver as opposed to sticking his hand in fire. However, the angel, the Yetzer Hatov, told Moshe that in fact the gold and silver is a "fire" which is even more dangerous than burning hot coals. For they can bring a person to sin which is even a stronger "fire"!
The Gemorah says that if a person finds shatnez in his clothing he should remove it even in the middle of the market place. The question is asked: How can it be expected of a person to bring upon himself such a terrible embaressment? The answer is quite simple: If someone was in the midst of a crowd of people and he noticed suddenly that his clothes caught on fire, would he leave his clothes on to avoid embaressment? Certainly not. Chazal knew and felt that each sin is "fire" to a person. As such, if he is wearing shatnez, i.e.: his clothes are burning, so to speak, he must remove them immediately, even in the market place!
Chag Kasher V'sameach