Answer

See John 8:57-58, which states, “The Jews then said to him [Jesus], ‘You are not yet fifty years old, and have you seen Abraham?’ Jesus said to them, ‘Truly, truly, I say to you, before Abraham was, I am.’”

The Jews, being familiar with the Old Testament, understood Jesus to be stating that he was God. Compare his statement to God’s in Exodus 3:14: “God said to Moses, ‘I AM WHO I AM.’ And he said, ‘Say this to the people of Israel, “I AM has sent me to you.” ’”

For more, see the Catholic Answers tract The Divinity of Christ.