From what I've been told, it's an old British name. Spelled "St John" but pronounced "Sinjin". It's not common in the UK, but they've heard of it.
After I heard it on the news, the first time in my life, I kept hearing it again!
I saw it in a Bond film, 007 was undercover and when his name was read aloud they mis-prounounced it Saint John. He then corrected them in his sultry way, Smithe, 'Sinjin' Smithe.
It was the same name the new priest had such a hard time pronouncing in Four Weddings and a Funeral.