The Scriptures are one book in Christ

“In the beginning was the Word, and the Word was with God, and the Word was God.”   – John 1:1

Irenaeus writes from the 2nd century: 
“If one carefully reads the Scriptures, he will find there the word on the subject of Christ and the prefiguration of the new calling. He is indeed the hidden treasure in the field — the field in fact is the world — but in truth, the hidden treasure in the Scriptures is Christ. Because he is designed by types and words that humanly are not possible to understand before the accomplishment of all things, that is, Christ’s second coming.”Origen writes from the 3rd century: 
“[Christ’s words] are not only those which he spoke when he became a man and tabernacled in the flesh; for before that time, Christ, the Word of God, was in Moses and the prophets. …[their words] were filled with the Spirit of Christ.”

Hilary of Poitiers writes from the 4th century: 
“Every part of Holy Writ announces through words the coming of our Lord Jesus Christ, reveals it through facts and establishes it through examples. ..For it is our Lord who during all the present age, through true and manifest foreshadowings, generates, cleanses, sanctified, chooses, separates, or redeems the Church in the Patriarchs, through Adam’s slumber, Noah’s flood, Melchizedek’s blessing, Abraham’s justification, Isaac’s birth, and Jacob’s bondage.”

Augustine of Hippo writes from the 5th century:
“You recall that one and the same Word of God extends throughout Scripture, that it is one and the same Utterance that resounds in the mouths of all the sacred writers, since he who was in the beginning God with God has no need of separate syllables; for he is not subject to time.”

“The Scriptures are in fact, in any passage you care to choose, singing of Christ, provided we have ears that are capable of picking out the tune. The Lord opened the minds of the Apostles so that they understood the Scriptures. That he will open our minds too is our prayer.”