Note 2 at Acts 16:17: Everything that this girl said about Paul and his companions was true. How could a demon possessed person magnify the truths that they were preaching?

She was doing this in mockery. It was well known to everyone in that region that this girl was possessed by spirits. To have her endorse Paul's message was not good. It led people to believe that Jesus and these demon spirits were co-operating.

This is a good illustration that not every spirit that speaks the right words is of God. When the scriptures say, "every spirit that confesseth that Jesus Christ is come in the flesh is of God" it is talking about more than words, it means lifestyle (see note 1 at Mk. 1:24, p. 117). One of the biggest "turn offs" to the gospel is those who say the right things but don't live it.