An Indiana priest tasked with the tough but necessary job of casting out demons from the possessed reports a skyrocketing demand for exorcisms, reports RTV6.

Father Vincent Lampert, one of 50 exorcist priests in the US, says his phone has been ringing off the hook: he gets 20 calls or emails every week from people asking him to perform the spiritual purge.

However, only 10 percent of interested callers needed evil spirits expelled from their person, he says. Four key signs helped him determine whether an exorcism is needed:

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Understand languages you don’t know

Abnormal strength

Extreme knowledge

Aversion to sacred items

‘Extreme knowledge’—the true mark of Satan.

Your average exorcism just requires intense prayer. But the priest has had a few brushes with total demonic possession, which is more work.

“The manifestations would include things like a person’s eyes rolling back in their head, foaming at the mouth,” Lampert told RTV6.

“There are different ways evil comes in. Somebody being cursed, a life of habitual sin,” the priest warns. But the big picture reason for the rise in evil spirits bent on invading their human hosts is the loss of faith in the Catholic Church.

“Faith in God will lead us in one direction and lack of faith will lead us in another, so there does seem to be a correlation with people who believe they are experiencing evil in their life at the same time where faith in God is less relevant,” he told the local TV station.

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Lampert is right about the declining popularity of the Catholic Church in the US. A 2015 Pew forum poll found that the Church has lost as many as three million members between 2007 and 2014. The rest are getting older: the medium age of adults who identify as Catholic was 49.

Catholicism’s US decline parallels a decline in Christianity more generally, with many younger people identifying as having no religious affiliation. But maybe exorcisms are just the thing to make religion relevant to young people again.