When we don’t know if one or another thought is from God or from the enemy, we should turn to Christ in prayer so as to get rid of the turmoil in our mind. Let us say, “O Lord, drive away this work of the enemy from my mind!” And when we know for sure that these are sinful thoughts, we should not attach importance to them. Elder Paisios the Hagiorite said the following about this, “Evil thoughts are like airplanes looking for airports. They are flying, but they must not land.” Like the seed of a weed, if you don’t allow it to fall on the earth of your soul, it can’t bear fruit. But if it has germinated, and you pour more water on it, it will take root.
The same can be said about thoughts in our mind. If we brood over them, they gain power over our souls and direct our being towards themselves. This is how sin is born. This is why we must face evil thoughts in time. Otherwise a little spark will set the whole forest on fire.