Also see the New Testament, Old Testament quotations in the New Testament, Old Testament Apocrypha.
The Old Testament shares books and canonicity with the Hebrew Bible.
The Old Testament is still worthy today
New Testament writers constantly quote the Old Testament (Old Testament quotations in the New Testament), so you cannot avoid reading it anyway.
- Romans 15:4 - For whatever was written in former days was written for our instruction, that through endurance and through the encouragement of the Scriptures we might have hope.
- 1 Corinthians 10:11 - Now these things happened to them as an example, but they were written down for our instruction, on whom the end of the ages has come. (speaking of Old Testament examples of disobedience like idolatry, sexual immortality, putting Christ to the test, and grumbling)
Jesus came to fulfill the Old Testament (known in the Hebrew Bible as the Law and the Prophets). All prophecies about Him will be fulfilled.
- Matthew 5:17-20 - Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished. Therefore whoever relaxes one of the least of these commandments and teaches others to do the same will be called least in the kingdom of heaven, but whoever does them and teaches them will be called great in the kingdom of heaven. For I tell you, unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven.
- Luke 16:16-17 - “The Law and the Prophets were until John; since then the good news of the kingdom of God is preached, and everyone forces his way into it. But it is easier for heaven and earth to pass away than for one dot of the Law to become void."
- Luke 18:31 - "everything that is written about the Son of Man by the prophets will be accomplished"
- Luke 22:37 - "For I tell you that this Scripture must be fulfilled in me:" ... (Jesus then quotes Isaiah 53:12) ... "For what is written about me has its fulfillment.”
- Acts 24:14 - But this I confess to you, that according to the Way, which they call a sect, I worship the God of our fathers, believing everything laid down by the Law and written in the Prophets,
- 2 Timothy 3:16 - All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness,
The apostles and preachers in the earliest days of Christianity only had the Old Testament to proclaim Jesus as the divinely sent Messiah, Lord, and Savior. John 5:39, "...These are the very Scriptures that testify about me."
(The Origin of the Bible: Newly Updated by F. F. Bruce, J. I. Packer, Philip W. Comfort, and Carl F. H. Henry, 2020. Biblical Languages by Larry Walker. Section One: The Authority and Inspiration of the Bible. Page 5-6)