Satan had deceived Adam and Eve our parents and let them commit the first sin against God. Quran 20:120 But Satan whispered evil to him: he said, "O Adam! shall I lead thee to the Tree of Eternity and to a kingdom that never decays?"
He let them out of heaven. Quran 2:36 Then did Satan make them slip from the (garden), and get them out of the state (of felicity) in which they had been. We said: "Get ye down, all (ye people), with enmity between yourselves. On earth will be your dwelling-place and your means of livelihood - for a time."
Satan continued his work in earth making all of us enemies. Even between members of same family. He made Joseph brothers envoy him and put him in a well. After a lot o sufferings in Egypt God put him in very high position close to the king and he brought his family to Egypt. Quran 12:100 And he raised his parents high on the throne (of dignity), and they fell down in prostration, (all) before him. He said: "O my father! This is the fulfillment of my vision of old! Allah hath made it come true! He was indeed good to me when He took me out of prison and brought you (all here) out of the desert, (even) after Satan had sown enmity between me and my brothers. Verily my Lord understandeth best the mysteries of all that He planneth to do, for verily He is full of knowledge and wisdom.
Satan continues to deceive man to commit sins, be enemy to other fellow man and to say what is wrong about God ( no God, Multiple Gods, blasphemies about God....). Quran 35:6 Verily Satan is an enemy to you: so treat him as an enemy. He only invites his adherents, that they may become Companions of the Blazing Fire.
In the day of judgment Satan will dump man after he took him to the wrong path through most of his life time.Quran 14:22 And Satan will say when the matter is decided: "It was Allah Who gave you a promise of Truth: I too promised, but I failed in my promise to you. I had no authority over you except to call you but ye listened to me: then reproach not me, but reproach your own souls. I cannot listen to your cries, nor can ye listen to mine. I reject your former act in associating me with Allah. For wrong-doers there must be a grievous penalty."
God will reproach us at that day because he had already warned us. Quran 36:60 "Did I not enjoin on you, O ye Children of Adam, that ye should not worship Satan; for that he was to you an enemy avowed?- Thus we have to get wise and understands a lot of evil thoughts and acts are inspired by Satan. He is our enemy and we should take him an enemy. He does not knock our door and say hi I am Satan let me in to deceive you. He enters through our normal thought process and threw an evil idea. When we build well our conscience mostly from religious upbringing we reject early any evil thought that cross our mind.