What was the relationship between Elijah and John the Baptist?
Malachi announced that God would send Elijah. Jesus said that John the Baptist was that Elijah who was to come. Malachi 3:23; Matthew 11:14; Matthew 17:12-13
What the Bible says
Matthew 11:14
And if you will receive it, he is Elias that is to come.
Luke 1:17
And he shall go before him in the spirit and power of Elias; that he may turn the hearts of the fathers unto the children, and the incredulous to the wisdom of the just, to prepare unto the Lord a perfect people.
Douay-Rheims Bible (Challoner revision), public domain.
What the Church teaches
The Catechism says that John is “Elijah who is to come”: the fire of the Spirit makes him run ahead of the Lord.
Catechism 718 · Catechism of the Catholic Church
Did you know?
When they asked John whether he was Elijah, he said no. John 1:21
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Meet Elijah and John the Baptist.