The Seven Sons of Sceva

Act 19:13 Then certain of the vagabond Jews, exorcists, took upon them to call over them which had evil spirits the name of the Lord Jesus, saying, We adjure you by Jesus whom Paul preacheth.

Act 19:14 And there were seven sons of one Sceva, a Jew, and chief of the priests, which did so.
Act 19:15 And the evil spirit answered and said, Jesus I know, and Paul I know; but who are ye?
Act 19:16 And the man in whom the evil spirit was leaped on them, and overcame them, and prevailed against them, so that they fled out of that house naked and wounded.

The seven sons of Sceva did not have a personal relationship with Jesus Christ. They were trying to use the name of Jesus through the name of Paul. "We adjure you by Jesus whom Paul preacheth", but they were overcome by this demon possessed man who said "Jesus I know, and Paul I know; but who are ye?" The power of the name of Jesus is not to be used as some magic incantation. The demonic man overcame them and "they fled out of that house naked and wounded." We cannot be second hand Christians riding on the coattails of someone else's faith.

The Apostles had been given authority to cast out demons by Jesus, but even they were not able to cast out a strong demon in a child.

Mar 9:28 And when he was come into the house, his disciples asked him privately, Why could not we cast him out?
Mar 9:29 And he said unto them, This kind can come forth by nothing, but by prayer and fasting

The power is entirely of God, not of man. Man must fast and pray to remove the influence of the flesh. Titles, pride and personal efforts are no good, the power is given through the authority of Jesus.

Mat 28:18 And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth.

Act 1:8 But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.

The power is through salvation by faith in Jesus Christ and receiving the Holy Ghost, followed by fasting and prayer. Do not be casually commanding demons to come out of people. The demons know Jesus and they know the power of the Holy Spirit.

Zec 4:6 ... Not by might, nor by power, but by my spirit, saith the LORD of hosts.

Get close to the LORD with prayer and fasting and obey him, do not attempt to cast out demons for a thrill or personal power; you may be spiritually naked. As a child of God, Holy Ghost filled and a person of prayer, do not be looking for demons, nor should you fear them, however, because of the authority you have in Christ, the demons will flee at the name of Jesus. We should always be ready to be used of God. To God be the glory.