CCC 497 The Gospel accounts understand the virginal conception of Jesus as a divine work that surpasses all human understanding and possibility:1 “That which is conceived in her is of the Holy Spirit”, said the angel to Joseph about Mary his fiancee.2 The Church sees here the fulfillment of the divine promise given through the prophet Isaiah: “Behold, a virgin shall conceive and bear a son.”3

CCC 712 The characteristics of the awaited Messiah begin to appear in the “Book of Emmanuel” (“Isaiah said this when he saw his glory,”4 speaking of Christ), especially in the first two verses of Isaiah 11:
There shall come forth a shoot from the stump of Jesse,
and a branch shall grow out of his roots.
And the Spirit of the LORD shall rest upon him,
the spirit of wisdom and understanding,
the spirit of counsel and might,
the spirit of knowledge and the fear of the LORD.5

1 Mt 1 18-25; Lk 1:26-38.
2 Mt 1:20.
3 Is 7:14 (LXX), quoted in Mt 1:23 (Greek).
4 Jn 12:41; cf. Isa 6-12.
5 Isa 11:1-2.