If I may be of some help to you here is the explanation for your query.
First, it was about False prophets, whom Jesus will try on the judgment day when they claim that they did miracles in His name, that He will say to them that He never knew them; nor did he approve of their works.
These references will help in understanding better:
Psalms 1:6; 2 Timothy 2:19; 1 Corinthians 8:3 and 1 John 2:19
Second, it is not clear about whom it was spoken of, but the reference is probably of one of the disciples of John or one of the seventy, whom Jesus sent. In all probability he was using the name of Jesus for his personal gains, but he was not an exorcist. Therefore, Jesus was saying that such work should not be stopped.
Third, it was about a reference to Beelzebub, who is commonly known as the one highly placed in Hell's hierarchy, by whose power Jesus is alluded to have been casting away demons. But Jesus was casting away demons by His own power as the Son of God:
Matthew 12:22 Then was brought unto him one possessed with a devil, blind, and dumb: and he healed him, insomuch that the blind and dumb both spake and saw.
23And all the people were amazed, and said, Is not this the son of David?
24But when the Pharisees heard it, they said, This fellow doth not cast out devils, but by Beelzebub the prince of the devils.
25And Jesus knew their thoughts, and said unto them, Every kingdom divided against itself is brought to desolation; and every city or house divided against itself shall not stand:
26And if Satan cast out Satan, he is divided against himself; how shall then his kingdom stand?
Hope this helps.