This month the Christian pastor Youcef Nadarkhani marked his 1,000th day of incarceration in Lakan, a notorious prison in northern Iran. Charged with the crime of apostasy, Mr. Nadarkhani faces a death sentence for refusing to recant the Christian faith he embraced as a child. He embodies piety and represents millions more suffering from repression—but his story is barely known.

Mr. Nadarkhani's courage and the tenacity of his supporters, many of them ordinary churchgoers who...