The Burning Bush and the Virgin Who Bore the Fire
Old Testament: Exodus 3:2-3
And the Lord appeared to him in a flame of fire out of the midst of a bush: and he saw that the bush was on fire and was not burnt. And Moses said: I will go and see this great sight, why the bush is not burnt.
New Testament: Luke 1:35
And the angel answering, said to her: The Holy Ghost shall come upon thee, and the power of the most High shall overshadow thee. And therefore also the Holy which shall be born of thee shall be called the Son of God.
The connection
Moses turns aside for "this great sight": a bush ablaze with the Lord's own fire, yet "not burnt" (Exodus 3:2-3), matter bearing the uncreated flame without being consumed by it. The bush does not generate the fire and does not survive it by its own strength; it simply holds what it was never able to contain on its own terms, and remains itself, unharmed, precisely because it is God who chooses to dwell there without destroying His vessel. Gabriel announces the same impossibility to Mary: "the Holy Ghost shall come upon thee, and the power of the most High shall overshadow thee" (Luke 1:35), the divine fire entering a created vessel who bears it whole and entire, and is not consumed but, unlike the bush, is not left as she was before either. Tradition has named her by this very image: a bush that burns and yet lives, becoming the pattern for a virgin who conceives and yet remains a virgin, the created nature bearing, without rupture, what it could never have produced on its own.
Moral sense
The bush contributes nothing to the fire except its willingness to stand where God chooses to appear; Mary contributes her whole free consent, and her own flesh, to a conception she does not produce. The soul's part in bearing God is never generation but availability, the same fiat offered in whatever small way grace is asking to dwell.
Anagogical sense
What the bush and Mary both show in miniature, a creature bearing divine fire and not being destroyed by it, is the destiny promised to every soul: to be "made partakers of the divine nature" (2 Peter 1:4), not consumed by glory but, at last, completed by it.
Catechism of the Catholic Church, paragraph 724.