Satan and his power
The Bible tells us that Jesus has crushed Satan's head (Gen. 3:15, John 19:28). Unless I'm misunderstanding something (in which case, please correct me), that means that the devil is now powerless. If that is so, then why do we consider him to be one of the three main forms of temptation (along with our sinful flesh and this world; see 1 Cor. 7:5 for one place Paul says that we can be tempted by the devil)?
Your question takes us back to the Garden of Eden. After sin shattered the perfection of Eden, God promised a Savior (Genesis 3:15). We will need to recognize the setting and the language to understand that Satan is still a powerful tempter today.
God spoke the words of Genesis 3:15 to Satan, but he spoke them for the benefit of Adam and Eve and all their descendants. God announced that there would be animosity between his children and those on the side of Satan—whether they are fallen angels or people.
God then revealed that one of Adam and Eve’s offspring, the promised Messiah, would crush Satan’s head. Keep in mind that Satan is a spirit being, having no body of flesh and bones. In Eden, he assumed the form of a serpent. With no physical head to be crushed, God’s promise spoke of a crushing blow to the power of Satan.
Jesus delivered such a blow when he rose triumphantly from the dead and then descended into hell to prove to Satan who had won the battle (1 Peter 3:18-20).
What we can say from Scripture is that Satan is a defeated enemy of God at this point. He still has the ability to tempt people today, and he seeks to do just that (1 Peter 5:8-9). God, in his wisdom, before the end of time will allow Satan to have more freedom to do his evil work (Revelation 20:3). But finally, on Judgment Day, Satan’s power will be crushed completely and he will not be able to tempt people anymore (Revelation 20:10).
In summary, God promised that a Savior would crush the devil “in respect to his head” (a literal translation). Jesus came into the world to “destroy the devil’s work” (1 John 3:8). Jesus dealt Satan a crushing blow when he resisted his temptations, died for the sins of the world and then rose to life victoriously. Jesus will put Satan out of commission on the last day.
I will explore your question a little more in a future “Light for our path” column in Forward in Christ.