1420 MHz (21 cm) is the hydrogen line — the emission frequency of neutral atomic hydrogen, and the most-listened-on frequency in the history of SETI. It is widely known as the "waterhole" frequency; in popular SETI writing it has sometimes been called "the voice of God" — the signal any intelligence in the galaxy would be expected to recognise.
Expanding on the above, the famous Wow! signal - an unexplained radio signal detected in 1977 that is hypothesized to have an extraterrestrial origin - had a frequency of ~1420.455 MHz.
Pairing "prophecy" with the hydrogen line reads the cosmos as a source of religious revelation: a transmission from elsewhere, listened to with the ear of a prophet rather than an engineer. Continues the band's long interest in number stations, shortwave, and hidden messages (see "Gyroscope").
Thematically paired with "Hydrogen Helium Lithium Leviathan" (the elements produced by the Big Bang) — together they stage the physical cosmos as a theological space.