It's clear to me.
And 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: And if Satan cast out Satan, he is divided against himself; how shall then his kingdom stand? Matthew 12:25-26
Jesus was accused of casting out demons by Beelzebul by the Pharisees which is why Jesus said "If satan cast out satan.."
These "sons of he Pharisees" were not casting out demons by Beelzebul either according to Jesus. So, it was not satan against satan. Nowhere did Jesus say demons only come out by the Spirit of God. ( that may be true but it is not said that in the text itself )