The last of five given Easter antiphons for the Rite of Sprinkling is an old, traditional text. The allusion is clear: the soldier plunging his spear into the side of the dead body of Jesus and from there, acclaiming him as Son of God.
Here is the text as given in the Roman Missal:
From your side, O Christ,
bursts forth a spring of water,
by which the squalor of the world is washed away
and life is made new again, alleluia.
Here’s an earlier version of this text set by J Michael Thompson. The acclamation had a somewhat different wording in earlier versions of the Missal.
If I were considering a setting of this, I might look to using the antiphon with one of the Christological New Testament canticles, Ephesians 1:3-10 perhaps. The 66th might be a prime choice from the Psalter.