Can demons go into Churches?

And no marvel; for Satan himself is transformed into an angel of light.  Therefore it is no great thing if his ministers also be transformed as the ministers of righteousness; whose end shall be according to their works.

2 Corinthians 11:14-15

Yes, I would say they are in a lot of Churches, even leading them.

2 Peter 2:1-2 KJV  But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction.  2And many shall follow their pernicious ways; by reason of whom the way of truth shall be evil spoken of.

The greatest threat to the success of the Churches ministry is from within ourselves. 

Acts 20:30 KJV Also of your own selves shall men arise, speaking perverse things, to draw away disciples after them.

"...the acquisition of such absolute command over the consciences and understanding of a congregation, however obscure or despised by the world, is more truly grateful to the pride of the human heart, than the possession of the most despotic power, imposed by arms and conquest on a reluctant people". 

Edward Gibbon, Esq; "History Of The Decline And Fall Of The Roman Empire",
Chapter XV: Progress Of The Christian Religion