Just read the article. As a believer of JC and the Bible, I would have to partially agree with the catholic priest though I'm not a catholic. I have conditioned my mind (well as much and often as possible), to always fill it with God, the Holy Spirit, Jesus' teachings etc. There are times when I empty my mind, madaming tukso. And sometimes, the temptation may come from my creative mind or maybe from the devil himself. We may or may not know. But it is also "possible" that it may come from the devil. No exact rule of the thumb.
I have a friend who is now a medical doctor, who used to be a member of the Ananda Marga, a religious group that practices tantra yoga. He says meditation requires you to empty your mind. Since it's impossible to think of nothing while meditating, the technique is to concentrate on a mantra, so that it's easier to block all other thoughts out.
He says "om" is a universal mantra, meaning it's a simple all-purpose mantra that works for everyone, which is why it's the mantra used by beginners.
For more advanced students, the guru assigns a personal mantra that can only be used by the person to whom it was assigned. It's usually a very long mantra, and you're not allowed to reveal it to anyone else. You're not even allowed to write it down. Kinuwento niya sa akin noong member pa siya, nahihiya daw siya sa guru, napakahaba ng mantra niya, nakakalimutan daw niya noong una, kaya pinapaulit pa niya sa guru during the next meditation sessions.
Tama na ang "om" ay isang yogic tool for emptying the mind. But it's not true that the devil can possess you while your mind is empty. Para kang robot na gagawin mo ang inutos sa iyo? That goes against free will.
For example, may galit ka sa isang tao. The devil reinforces your own hatred and tempts you to kill him. The devil tempts you to kill by deceiving you into believing that killing is justified in this case. Tapos pinatay mo nga.
So the evil thoughts actually originated with you, and the devil only tempted you to act. So you still killed out of your own free will. Hindi yung nag-meditate ka lang, tapos nung nagising ka, bigla kang nagdecide na patayin si Pedro...
The idea is nothing new. I think it first became popular after the movie "The Exorcist" was released. Delikado daw ang Ouija board, because it opens you up to demonic possession. Sumunod, noong nauso ang "TM" (transcendental meditation), ang sabi naman, yoga and meditation open you up to demonic possession.
These days, ang uso, New Age practices will open you up to demonic possession.