There are two verses in the Bible I use quite frequently. One of them is First John 3:8, mainly the second part.

But here is the whole verse according to the New International Version:

“He  who does what is sinful is of the devil, because the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the devil’s work.” 

WHAT IS SATAN’S WORK?

1. Matthew 13:25,30: “But while everyone was sleeping, his enemy came and sowed weeds among the  the wheat, and went away.”

He is sowing weeds and wrong doctrine.

2. Luke 22:31: “‘Simon, Simon, Satan has asked to sift you as wheat.'”

He is testing the Christians.

3. First Corinthians 5:5: “hand this man over to Satan, so that the sinful nature may be destroyed and his spirit saved on the day of the Lord.”

His work is to destroy the flesh (lives) of men.

4. Second Corinthians 4:4: “The god of this age has blinded the minds of unbelievers, so that they cannot see the light of the gospel of the glory of Christ, who is the image of god.”

He is blinding the minds of men.

5. First Peter 5:8: “Be self-controlled and alert. Your enemy the devil prowls around like a roaring lion looking for someone to devour.”

He is seeking whom he may devour.

6. Revelation 12:10: “For the accuser of our brothers, who accuses  them before our God day and night, has been hurled down.”  

He is accusing the brethren.

The second verse I use often you find in the last book of the Bible, in the book of Revelation: In Revelation 12:11 it says:

 “They overcome him by the blood of the lamb and by the word of their testimony; they did not love their lives so much as to shrink from death.”