When, like many other passengers, three American friends boarded a train from Amsterdam to Paris on August 21, 2015, they could not have known that they would disembark as international heroes.

But after a gunman opened fire, causing panic and mayhem, in the moments they decided to act it would all make sense. The assailant, Ayoub El Khazzani, maintains he was merely intent on committing robbery, but he was branded a terrorist by French police and the world's media.

Reflecting now, Anthony Sadler, Oregon National Guardsman Alek Skarlatos, and US Air Force Airman First Class Spencer Stone – all committed, church-going Christians – thank God for their joint presence on that fateful early evening.

It would seem that their lives had led them to be together on what has now become a movie title, The 15:17 to Paris, directed by Clint Eastwood, made by Warner Bros and released in the US on February 9, starring the men who play themselves.

Based on the book The True Story of a Terrorist, a Train, and Three American Soldiers by Jeffrey E Stern, the film weaves in biography of the trio, who met at a Christian school, with the dramatic scenes from the train itself.

Speaking by telephone to Christian Today, the friends, all still in their twenties, unashamedly see God's hand in their ability to tackle and subdue the gunman that day.

'We see this as evidence of God's existence because of so many things that happened that day,' says Alek. 'The odds are too astronomical to be just chance alone.'

Anthony agrees. 'It's kind of like our lives were leading up to that moment. You don't always know what plan God has for you. What we've come to realise with hindsight is that we saw [as coincidences] were all part of a plan, a bigger picture. That's where we were supposed to be that day.'

Asked about the role of free will, Spencer says: 'I would say God gives you what you need to do what's in front of you, and it's really up to you to make the choice to act or not.'

He confirms that he was scared. 'Yeah, we were all pretty frightened. It's a pretty paralysing feeling – you look at a guy with a loaded automatic AK47 and you think the only result is going to be your death. Running at him, we had no hope of actually making it to him. When I made it to him, I was shocked. But yeah we were all pretty scared that day.'

Did they have time to pray? 'We didn't have time to do anything, it was act or die,' says Alex. 'There wasn't any time to think about anything because if you didn't do anything you'd end up dead, and we just kind of went for us because we expected to die. With hindsight, I guess I was prepared to die – but we all thought we were going to die anyway, we were on a moving train, nowhere to go.'

Nonetheless, they maintain there is no trauma from the events three years ago, and their faith has been strengthened. The men say that the incident has provided 'confirmation' that 'someone is looking out for us'.

The second being that they credit is Eastwood, of whom they are in awe and whose movie has led them all to want to pursue a career in acting. Spencer explains: 'I got out of the air force a little over a year ago, and we've pretty much been working on the film ever since I got out. Having this experience of being able to play ourselves gave us the acting bug. The two months filming the movie was the funnest two months of our lives. If we can make it a career, why not? So I think we're all going to give that a shot.'

Alek concurs: 'I got out of the army in November and I'm also trying to get into acting and see where that takes me.'

And Anthony says: 'I just graduated from university in May of last year, been working on the film ever since and like the guys said we're all trying to pursue it. We [don't want] to waste this opportunity.'

Do they have faith that their friendship will be lifelong? 'Absolutely,' says Spencer. 'We'll always be friends. Everything we've been through together, there's no separating us at this point. That's the great thing about our friendship – we can go off and do our own thing, and maybe not even talk for a long time, but there's no ever questioning our friendship. It will always be there, and we can pick up right where we left off at any time. We've done that throughout our entire lives, and I don't see it changing.'

And ultimately, they are united by their Christianity, a faith to which they return again and again when talking about the movie and the journey on which it is based.

'I believe we're vessels, to be used by [God], to do his work. And it was an honour to do something that good,' says Spencer.

'I think it's our responsibility to take that message and be responsible with it and spread it as much as we can so we don't waste the opportunity that he gave us,' explains Anthony, the son of a pastor. 'We're meant to spread the story and it's meant to touch people.'