The hymn begins with the line "Come, ye faithful, raise the strain," and, as if to reflect that "rais[ing]," the line is sung to an ascending melody (spanning a whole octave, even).
This hymn also appears (as "Come, You Faithful, Raise the Strain") in The Lutheran Service Book (#487), although the text is a bit different and sung to a different tune ("Gaudeamus pariter"). There's a similar feature in that version, but it's not as extensive as what's shown above, where the entire phrase ascends.