Sacred music isn’t particularly a part of my life. But I spent 15 years singing it almost every day. I was a chorister at St Paul’s Cathedral from the age of 9, then I went to my senior school as a music scholar and then to Gonville and Caius College, Cambridge, as a choral scholar.

For two hours a day at St Paul’s you’d be treated as a professional, which is extraordinary, really, when you think about it. I don’t remember learning to sight-read music, for example – we were taught theory, and the oboe in my case – but then in a year you’d be sight-singing the most incredibly complicated stuff.

When you watch choirs there’s that thing they do when a singer makes a mistake – they put their hand up. It’s an acknowledgement that you know you’ve made a mistake and you don’t have to go over it again. It became so inbred that even now I do it in rehearsals for a play.

As a chorister I never felt under pressure. I think that was why it was rather magical, because you just absorbed information by osmosis and did your job. I was never given any solos, though. As an adult I had a slightly better voice, though I never felt a natural tenor.

When I hear the first chords of Bach’s Matthew Passion it still has a huge great thrill for me. At St Paul’s we very rarely performed it with an orchestra, and it’s the sheer thrill of having the band play that magnificent first movement that I remember. So when it came to singing a Bach motet with Harry Christophers and the Sixteen for my new BBC Four series Sacred Music, it was simply a thrilling experience.

Even when I left Cambridge and went to a singing course at Guildhall, the acting lark was always at the back of my mind. And I would never have made a successful career as a singer: my heart wasn’t really in it. I’ve droned for the Sixteen and that’s about my level now.

Some of the music we explored in the series I knew nothing about, such as the 12th-century music from tonight’s first episode. I hadn’t really thought about how extraordinary it must have been to be in Notre Dame when composers such as Léonin and Pérotin started subdividing the musical line. And watching the sheer effort the guys in the Sixteen put into singing that stuff was extraordinary too. I’d assumed that if you sang 12th-century music it was probably quite easy, but it’s a real physical effort.

When it came to more familiar music, Byrd and Tallis, I knew that Byrd had been a Catholic but I was unaware of how skilfully he manipulated his world to survive as a Catholic composer – and how lenient Elizabeth I was about it.

In fact what was interesting about the Tudor programme was the whole issue of the politics of the worlds in which these composers were working. It’s also about defining what we think is good sacred music. That opens up a huge can of worms: Rossini’sPetite Messe Solennelle, Verdi’s Requiem and even some Mozart Masses, with concert arias in the middle of them, push at the boundaries of what we accept as “proper” sacred music. The composers we looked at had to find a method where they could compose and still fulfil the Church’s injunctions. Palestrina or Bach did it exremely well – but could they have created greater masterpieces?

Making Sacred Music was a very personal journey. The highlights were lots of little things, such as discovering the library of Wolfen-büttel. You take the night train from Paris to Berlin and then you stop off at this tiny town, the old capital of some tiny German dukedom, where there is an international library of extraordinary importance. It has the first Martin Luther Bibles and manuscripts of 12th-century music.

The subject matter of Sacred Music is a bit rarefied, but that’s no reason why it isn’t an interesting and important story. Whatever your faith, or lack of faith, there’s something very moving about the fact that what these composers regarded as their highest calling was the praise of God. There was always a more important reason for writing this music than the vanity of pleasure. I found that extraordinary, and when you think about Bach doing that year in, year out, it’s very moving. It has an extraordinary resonance, even to a secularist.

I was immersed in Sacred Music for seven weeks, so I got to see the stained glass in Sainte-Chapelle by myself, I got to climb to the top of Notre Dame and stand at the top of St Peter’s in Rome. That’s what I wanted to do – that was the carrot, if you like – that, and sit and listen to the beautiful music.

As I say, I’m a secularist, but like a lot of people I have a puzzlement about the spiritual dimension. There’s a bit of me that can’t believe that this music couldn’t have been written without some sort of literal inspiration from something else.

The play I’m doing at the moment at the National, Much Ado About Nothing, is one of the most human of Shakespeare’s plays, but it has a sort of spiritual kick. And when you get to the end of Twelfth Night and The Winter’s Tale you get into areas of redemption, forgiveness and grace. It’s the same effect, whether it’s the statue coming to life at the close of The Winter’s Tale or one of Bach’s motets. In The Winter’s Tale it’s about coming to a very human need to be forgiven and have the chance to try again. In Bach’s Singet dem Herrn it’s about joy, it’s about singing up to Heaven and it’s about peace; the peace of fulfilling the potential of your humanity.

Sacred Music starts tonight on BBC Four (8pm).

Explore Music

• Proms

• Festivals

• CD Reviews

• Live Reviews

Editor's choice

• The End: the worst movie endings ever
• Introducing The Times Proms Podcast
• Heath Ledger tipped for Oscar