Lesson of a demon beating 7 men

"Although the Jewish exorcists mentioned in the passage cited in the question invoked the name of Jesus, they themselves had no faith in Him (as indicated by the fact that they referred to Him as "Jesus, whom Paul preaches...."  TimMaas

A group of Jews was traveling from town to town casting out evil spirits.
They tried to use the name of the Lord Jesus in their incantation, saying, We adjure you by Jesus whom Paul preacheth.
And there were seven sons of one Sceva, a Jew, and chief of the priests, which did so.

And the evil spirit answered and said,

Jesus I know, and Paul I know; but who are ye?

 

The seven sons of Sceva lost a 7-to-1 fight:

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.
Acts 19:13-16
"The seven exorcists were not genuine followers of Jesus and so lacked the authority and power to heal in Jesus' name.
A painted fire provides no heat.
It is futile to:

(a) Preach without belief. (b) Sing gospel songs without belief.(c) Pray without belief." jfmreed