But each person is tempted when he is lured and enticed by his own desire. Then desire when it has conceived gives birth to sin, and sin when it is fully grown brings forth death.

Sin starts with a temptation that can be our own desires, contrary to God, leading to sin against God. The source is always with us and we must fight against it - its a daily battle. Our weapon against sin is the shield of faith with which you can extinguish all the flaming darts of the evil one. (Ephesians 6:16b) which allows us to stand firm and when we fall, seek forgiveness from the One who is just and righteous.