Zechariah 12:10 is one of the strangest verses in the prophets — the LORD Himself speaking: 'And I will pour out upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace, and of prayers: and THEY SHALL LOOK UPON ME, WHOM THEY HAVE PIERCED: and they shall mourn for him as one mourneth for an only son.' God pierced; God mourned for as a firstborn son — the grammar shifting between 'me' and 'him' as if the oracle itself strains at what it carries.
After Jesus' death, 'one of the soldiers with a spear opened his side, and immediately there came out blood and water' (John 19:34). John cites the verse as the second scripture of that hour: 'And again another scripture saith: THEY SHALL LOOK ON HIM WHOM THEY PIERCED' (19:37). The Apocalypse opens by universalizing it: 'Behold, he cometh with the clouds, and every eye shall see him, and they also that pierced him' (Revelation 1:7).
The Church's devotion to the pierced side — the blood and water read since the Fathers as baptism and Eucharist flowing from the sleeping New Adam, and in modern times the wellspring of the Sacred Heart and Divine Mercy devotions — is this prophecy's long afterlife. Zechariah's next verse promises what the looking accomplishes: 'In that day there shall be a fountain open to the house of David... for the washing of the sinner' (13:1).