Be especially attentive "to the content and unity of the whole Scripture". Different as the books which compose it may be, Scripture is a unity by reason of the unity of God's plan, of which Christ Jesus is the center and heart, open since his Passover.
The phrase "heart of Christ" can refer to Sacred Scripture, which makes known his heart, closed before the Passion, as the Scripture was obscure. But the Scripture has been opened since the Passion; since those who from then on have understood it, consider and discern in what way the prophecies must be interpreted.
Source: Catechism of the Catholic Church (Vatican.va)