For religious believers, being compelled by authorities to produce evidence "proving" one's faith is an old ordeal—at least as old as the pharaoh's demand that Moses and Aaron perform a miracle. But a latter-day example in the English courts serves mostly to prove that the law is better off steering clear of theology.

Judge Elizabeth Roscoe of the Westminster Magistrates' Court of London issued a summons in late January to Thomas Monson, the Utah-based president and prophet of the Mormon Church, to appear at a March...