Then He said to them, “These are the words which I spoke to you while I was still with you, that all things must be fulfilled which were written in the Law of Moses and the Prophets and the Psalms concerning Me.”
The Law, the Prophets and the Psalms designate the three sections of the Hebrew Scriptures we call the Old Testament.
The Holy Bible, Old and New Testaments, make up a seamless, unified whole. Jesus Christ inspired hundreds of Old Testament prophecies that He later fulfilled through His birth, life and death. And there are many more prophecies that will be fulfilled in His second coming.
The Old Testament actually revealed Jesus Christ (see "Jesus in the Old Testament?"), and Jesus Christ then revealed the Father in the New Testament (Luke 10:22; John 14:6-11).
After His resurrection, Jesus “opened their understanding, that they might comprehend the Scriptures” (Luke 24:45). Though much of the Bible is accessible and helpful on many levels, without God’s help we can’t fully comprehend the Scriptures. But with the help of God’s Spirit we can understand “the things of God” that are “spiritually discerned” (1 Corinthians 2:11, 14).
For more about how God opens our understanding of the Bible, see our article “How Do You Know You Have the Holy Spirit?”