You can see their activities in scripture. Everything from sickness to false teaching, to paralysis, to fortunetelling. All of those Jesus delivered were Christians. Even the Gadarene demoniac had 6,000 demons. They gain access through occult practices.
“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 19:16 (KJV 1900)
And as a result: “Many of them also which used curious arts brought their books together, and burned them before all men: and they counted the price of them, and found it fifty thousand pieces of silver.” Acts 19:19 (KJV 1900)
False teaching in churches: “Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils;” 1 Timothy 4:1 (KJV 1900)
And drug use: sorcery especially exercised through the mixture of substances to make potions; especially used of poison-making. Logos Fact Book.
“And certain women, which had been healed of evil spirits and infirmities, Mary called Magdalene, out of whom went seven devils,” Luke 8:2 (KJV 1900)