The beginning of the Gospel is in these words of the Angel to the shepherds: I announce to you, word for word, I evangelize you, I bring you the good news, which will be the subject of great joy, and it is is that of the birth of the Saviour of the world. What happier news than that of having a Saviour? He himself, in the first sermon he gave in the synagogue after leaving the desert, explains this subject of joy to us through the words of Isaiah, which he found at the opening of the book: The Spirit of the Lord is on me; therefore he consecrated me by his anointing; he sent me to preach the Gospel to the poor, and to bring them the good news of their deliverance; to heal those who have a grieving heart; to announce to the captives that they are going to be set free, and to the blind that they are going to receive sight: send back in peace those who are overwhelmed with evils; proclaim the year of mercy and forgiveness of the Lord, and the day when he will give good people their reward, as also punishment to others.
What joy like this could be given to men of good will, and what greater cause for joy? But at the same time is not the greatest subject to glorify God? And what can good people desire more than to see God exalted by so many wonders? So this is what the Gospel is: it is by learning the happy news of man’s deliverance, to rejoice in seeing there the greater glory of God. Let us rise to the high places, to the most sublime part of ourselves; let us elevate ourselves above ourselves, and seek God in himself to rejoice with the angels in the great glory.
It’s interesting to note that Bossuet implies that Jesus found the reading from Isaiah rather than having chosen it. This is discussed in the earlier post You Really Can Do Biblical Preaching From a Lectionary.