Mac Powell has been up all night. Well, he went to sleep at 6.30am after playing to a theatre full of fans but he's up and ready for an interview with me at 8am. Third Day are on tour, and the band that has been together for 24 years, sold eight million CDs, won four Grammys and more than 20 Dove awards (the Christian music industry's equivalent) are loaded on the tour bus and off to another gig, this time in Chattanooga, Tennessee.

I first met Mac in Thailand at an event for Christian leaders passionate about making a difference for vulnerable children, where he was leading our worship. What struck me about him was both his astounding voice and his astonishing humility. The former goes some way to explain why Third Day has sold so many records; the latter is totally inexplicable and inspirational. I saw Mac work alongside local musicians, lead a room in singing songs that the Thai musicians had written, and encourage them to take the lead while he sang harmonies. I witnessed him take part fully in the whole conference and not just camp out in a back room like so many worship leaders are tempted to do at these kind of events. With his huge voice and gentle spirit, Mac makes room for an interview with me from the Third Day tour bus as it storms its way down the highway towards their next gig, despite barely an hour of sleep.

KK: How did you know you were called to serve God in music?

MP: I had a reconversion experience in my last year of high school. I had been part of a church all of my life but it only really became personal to me as I realised I was about to finish high school and I had decisions to make about what I was going to do with my life. Then someone told me I should read the book of Romans. Even though I had been going to church my whole life I had to look up in the index where that was. But I found it and read it and something of the simplicity of the gospel struck me and I started a relationship with Jesus. My friend Mark Lee and I both became Christians at the same time and we decided that we wanted to share our faith, and music seemed to be the best way to do it. So we played as much as we could in garages and church youth groups. Gradually more and more invitations came and we made the big decision to quit college and do the music full time. The rest, as they say, is history...

What advice would you give someone feeling called into music ministry?

Don't quit college! I would also say pray and play. Pray for God's guidance but also play – as often as you can, in as many different places as you can, to test the calling.

You mentioned that although you had been in church you hadn't been discipled. What made the difference for you in high school?

Well our church didn't really have anything in the way of youth ministry but there was a guy who was just a few years ahead of us and he mentored my friend and me. For me it was needing to be grounded in the triple witness of the Word of God, the Spirit of God and people of God, and we tried to stay grounded with all three all the time. I still hold firm to these three things today.

I imagine staying connected with a church can be difficult with all the travelling and touring you guys do. How does church work for you?

Well when we are touring we bring along our British chaplain, Nigel. He helps us to make sure that when we are together we are not being "professional Christians", but living out our faith on the road too.

Third Day has been together for 24 years. What is it that helps you keep going?

Well the spiritual answer is Jesus and that is of course true. But the other reason is that we have gotten good at doing this and we can do some good through what we do, and also there's the fact that we can support our families through our work.

After all these years, how do you keep the music fresh?

Well we have never tried to be a cool band that followed the musical trends. We like Rock and Roll and so we have stuck with that. When I was a teenager I had two choices: the Beatles, Rolling Stones and Dylan on the radio or the music in our church, which was an organist playing an up-tempo version of 'Blessed assurance'. We want to make sure people have different opportunities today when it comes to worship.

What's the story behind the song 'Soul on Fire'?

I had just finished writing all the songs on our new album 'Lead us Back' and our producer said that we should go for another upbeat worship song. He suggested I listen to a track by the South African Christian singer-songwriter Brenton Brown. I didn't think so. But strangely it was another English connection that made me take another look. We were playing at the Big Church Day Out in Sussex and I gave it a listen, I ended up rewriting the verse and then played the whole thing to some friends and suddenly we realised we had a hit song on our hands. It is one of the most successful songs we have ever made. It was number one for 11 weeks!

I know the song went down really well in Thailand – it was amazing to have people from 60 different nations singing it out together.

Yes, the Thailand event was one of the most significant events in my life and I don't say that lightly. It was amazing to meet so many people that felt that same way we do about the importance of caring for vulnerable children. We have three birth children and we have adopted two children as well. My wife Aimee and I were high school sweethearts. We talked about adoption even before we got married. In fact, when we were dating we both helped out in an after-school programme which had many children from difficult backgrounds in it. We thought back then that adoption should be part of the way we do family. So when we had three birth children we decided to add to our family through adoption.

Some men I know are worried about adopting or fostering because they are not sure they could love a child they are not biologically connected to. What's your response to that?

To be honest that was my worry too. I thought it was the right thing to do to adopt a child in need. But I thought I might only be able to love a child 80 per cent of the amount I love my own birth children – which I thought was still better than leaving a child in need. It turns out that I feel very close to my adopted children. Something my mentor said to me really struck me. He reminded me that in the Bible adoption can't be undone. It is forever. That helped me grasp something powerful about the nature of God's adopting love for us, and as an adopted Dad now I understand at a deeper level how God feels about us. God is never giving up on us.

As I sign off on the call with Mac and he and the band speed on to their next sell-out gig, I am struck by how the conversation has come full circle. The book of Romans that brought Mac to faith has a lot to say about our adoption into God's family. Paul writes: "the Spirit you received brought about your adoption to sonship. And by him we cry: 'Abba Father'."Perhaps you are experiencing something of that in your own experience of being adopted into God's family, or through adopting children. My prayer is that through hearing Mac's story and the story of so many other families around the world, adoption would become a normal part of life for more and more people.