1. “John 25 or 6 to 4” references the famous song “25 or 6 to 4” by the rock band Chicago. The song title is often interpreted as referring to a time (25 or 26 minutes to 4 o’clock, i.e., 3:35 or 3:34 AM), though its meaning is ambiguous and tied to the creative process or drug-induced haze.

2. “Pope Leo cites John” sets up the expectation of a biblical reference, as “John” suggests the Gospel of John, and a pope citing scripture is plausible. However, there is no “John 25” in the Bible (the Gospel of John has only 21 chapters). The phrase “25 or 6 to 4” is humorously substituted for a nonexistent Bible verse, creating a surreal and absurd contrast.

The humor comes from the incongruity: the solemnity of a pope citing scripture is undercut by referencing a rock song, and the listener is left chuckling at the clever mashup of high culture (papal authority) and pop culture (a 1970s rock hit).

(http://www.autoadmit.com/thread.php?thread_id=5722842&forum_id=2E#48921205)