This text comes from Mediaeval Hymns and Sequences, edited by John Mason Neale, a historical songbook transcribed on Wikisource and reproduced from the pinned revision linked below.
The Royal Banners forward go;
The Cross shines forth in mystic glow;
Where He in flesh, our flesh Who made,
Our sentence bore, our ransom paid.
Where deep for us the spear was dy'd,
Life's torrent rushing from His side,
To wash us in that precious flood
Where mingled Water flow'd, and Blood.
Fulfill'd is all that David told
In true Prophetic song of old;
Amidst the nations God, saith he,
Hath reign'd and triumph'd from the Tree.
O Tree of Beauty! Tree of Light!
O Tree with royal purple dight!
Elect on whose triumphal breast
Those holy limbs should find their rest!
On whose dear arms, so widely flung,
The weight of this world's ransom hung:
The price of human kind to pay,
And spoil the Spoiler of his prey.
[O Cross, our one reliance, hail!
This holy Passiontide, avail
To give fresh merit to the Saint,
And pardon to the penitent.
To Thee, Eternal Three in One,
Let homage meet by all be done;
Whom by the Cross Thou dost restore,
Preserve and govern evermore.]