>How sweet a thought,
The concept of God's sacrifice is sweet to us, for it provides our souls with ultimate care through eternal life.
>How strange a deed,
When many of us were disbelieving, as selfish, wretched mortals, we viewed a sacrifice of our own life as strange, for many are too absorbed within their own flesh to think spending our own lives for even anything precious could be a proper act. Christ, being perfect, committed this deed, this gracious sacrifice of His blood, for our imperfect souls.
>To house such glory in a seed
Glory is housed within the literal seed of Christ that God provided to Mary.
>A berry, shining rufously,
Christ is represented through this berry, shining with ever-present glory through His amazing blood, which was spilled for all of our sakes.
>Like scarlet coral in the sea!
Simple, added emphasis of the redness of the blood upon Christ; extra words adding to the description of His glory. A connection is also formed between red blood and the sea, being water, in that Christ bled water upon his stabbing by Longinus. Blood took death away from us as water cleansed us to gain eternal life with God.
>A berry, rounder than a ring,
Christ is eternal, being everlasting God, and so God's existence is "circular", connecting beginning to end, because time does not have an effect on Him. His guidings and His commands are also as proper today as they were when they were given, so there is no disconnection; the start and the end connect through the concept of eternity, and so it forms, within analogy, a connected ring.
>So round, it harbours everything;
God's knowledge is ever-deep, forever extending. Christ harbours this knowledge because Christ is God. The eternity of God's existence in His natures means that His knowledge is also eternal, and since He is the absolute creator, He knows all.
>So red, that all the blood of men
All of our goodness combined will never amount to Christ's glorious perfection.
>Could never paint it so again.
As stated, we will never amount to the perfection of Christ.
>And, as I hold it in my hand,
Through the Holy Spirit, we have within us God, holding Him with us just as He holds us with Him through our faith in Him and His grace to us.
>A fragrance steals across the land:
Description of the spreading of the Gospel through the Holy Spirit within us. Fragrances spread with beauty in their scent, the Gospel spreads with beauty in its message.
>Rich, on the wintry heaven, I see
"Wintry heaven", thus being an existence of light like brightness of snow, and "rich" in its very existence as being with God eternally. Being in salvation, we will view...
>A white, immortal hawthorn-tree.
Hawthorn trees bring the hawthorn berry (represented in this interpretation as Christ), just as the Tree of Life on the New Earth bears us the healing of nations from its leaves (Christ healing all of our nations through his Gospel). This tree, like all things being of eternity, is eternal, thus immortal.