What was the relationship between Isaiah and John the Baptist?
Isaiah spoke of a voice crying out to prepare the way of the Lord in the desert. John the Baptist said that he was that voice. Isaiah 40:3; John 1:23; Mark 1:2-4
What the Bible says
Isaiah 40:3
The voice of one crying in the desert: Prepare ye the way of the Lord, make straight in the wilderness the paths of our God.
John 1:23
He said: I am the voice of one crying out in the wilderness, make straight the way of the Lord, as said the prophet Isaias.
Douay-Rheims Bible (Challoner revision), public domain.
What the Church teaches
The Catechism says that John is the “voice” of the Consoler who is coming.
Catechism 719 · Catechism of the Catholic Church
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Meet Isaiah and John the Baptist.