Turkey has been Ryan Keating's home for more than 20 years. But last week all that was thrown into jeopardy.

After a short trip out of the country, the American was told he could never come back. No reason was given. There was no investigation. No evidence. He was held in a cell overnight at Istanbul airport and interrogated by the anti-terror police. Then he was told he was a threat to national security and forced aboard the next plane out the country.

"Unfortunately this is typical of the kind of treatment that Christians often get in Turkey," he told Christian Today.

Keating has been in and out of the country since 1993 and has been a full-time resident in the capital Ankara with his wife and four children for the last ten years.

He is doing a PhD in philosophy of religion at Ankara University and has set up Ankara Refugee Ministry (ARM), which provides food, shelter and clothing to 6,000 of refugee families. Run out of Kurtulus Church, one of Turkey's largest evangelical churches, which the Keatings attend, ARM also offers English classes and career training for a handful of Turkey's 2.7 million refugees.

On top of that Keating runs a coffee company called Coffee Haus and directs a discipleship program at his church.

All that is at risk now because Keating has been labeled "a threat to national security" and given a lifetime ban from Turkey.

He flew to the UK for a short visit on October 8 and when he returned to Turkey on October 17 he was told his residence permanent had been cancelled. "They searched me and a terror division police officer interrogated me. I wasn't mistreated in that process but I was locked up overnight," he said in an interview with Christian Today.

"Then they put me on the next flight back to London Gatwick." The UK has given him a six month leave to stay in light of his situation but his wife and children are still in Turkey.

He has appealed the decision but cases such as his normally take at least two years to resolve and for that time, Keating is a nomad. But he is quick to stress his treatment is not unusual for Christians in Turkey.

"There has been some ways in which Turkey has given increased freedom to religious minorities. But there are other incidents of arbitrary discrimination and this is one of them.

"There is no evidence or justification for why I have been banned. I know I haven't done anything illegal ever in Turkey. We are very careful to obey the laws. We have done nothing to threaten or do harm to Turkey in any way.

"There has been no investigation, no evidence, just an arbitrary ban. And to use this blanket 'threat to national security' – what does that even mean? What are they suggesting I have done or would do?"

Turkey saw an attempted coup d'etat against its President Erdogan in July. But the failed revolution has just given licence to crackdown on religious freedom. "There has been an atmosphere of tension and fear and suspicion," said Keating.

He points to other missionaries he knows who have been expelled without explanation in the last few weeks. Patrick Jenson. Idris Kabil. David Bile. Just three names among more than 100 to have been banned.

"It's a tragedy," said Keating. "It's very sad."

Although his family have stayed in Turkey for the time being, they are concerned about their future. They have packed emergency bags in case they are arrested at short notice.

"We have generally been safe, if tense, in Turkey," he said. "But if I can be banned they're all worried about they will be as well."