Authority over demons

Demons still flee at the name of Jesus’ name – its power has not diminished. To ask in the name of Jesus Christ means:
... to ask in His merit. We come before God in Christ’s standing.
... I come as His representative. When an ambassador goes to another country, he does not go in his own name. He goes in the name of his own government... He is a representative. Paul in 2 Corinthians 5 said, “We are ambassadors for Christ, for God...”
Can a Christian be demon-possessed?