The Old Testament explains:
1. God’s purpose in creating the earth.
Isaiah 45:18 For thus saith the LORD that created the heavens; God himself that formed the earth and made it; he hath established it, he created it not in vain, he formed it to be inhabited: I am the LORD; and there is none else.
Numbers 14:21 But as truly as I live, all the earth shall be filled with the glory of the LORD.
2. Why we die.
Genesis 3:17 And unto Adam he said, Because thou hast hearkened unto the voice of thy wife, and hast eaten of the tree, of which I commanded thee, saying, Thou shalt not eat of it: cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life; 18 Thorns also and thistles shall it bring forth to thee; and thou shalt eat the herb of the field; 19 In the sweat of thy face shalt thou eat bread, till thou return unto the ground; for out of it wast thou taken: for dust thou art, and unto dust shalt thou return.
Ezekiel 18:4 Behold, all souls are mine; as the soul of the father, so also the soul of the son is mine: the soul that sinneth, it shall die.
3. God’s character
Exodus 34:6 And the LORD passed by before him, and proclaimed, The LORD, The LORD God, merciful and gracious, longsuffering, and abundant in goodness and truth, 7 Keeping mercy for thousands, forgiving iniquity and transgression and sin, and that will by no means clear the guilty…
4. God’s promise of a redeemer who would be a sacrifice for sin.
Isaiah 53:10 Yet it pleased the LORD to bruise him; he hath put him to grief: when thou shalt make his soul an offering for sin, he shall see his seed, he shall prolong his days, and the pleasure of the LORD shall prosper in his hand.
Hosea 13:14 I will ransom them from the power of the grave; I will redeem them from death: O death, I will be thy plagues; O grave, I will be thy destruction: repentance shall be hid from mine eyes.
5. God’s choice of a man (Abraham) and a nation (Israel) through whom the redeemer would come.
Genesis 22:18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.
6. God’s Covenant with Israel which is where the book gets its name – ‘Old Testament’ is in fact the ‘Old Covenant’.
Exodus 24:7 And he (Moses) took the book of the covenant, and read in the audience of the people: and they said, All that the LORD hath said will we do, and be obedient. 8 And Moses took the blood, and sprinkled it on the people, and said, Behold the blood of the covenant, which the LORD hath made with you concerning all these words.
7. God’s choice of a king (David) through whom the redeemer would come.
1 Chronicles 17:10… Furthermore I tell thee that the LORD will build thee an house. 11 And it shall come to pass, when thy days be expired that thou must go to be with thy fathers, that I will raise up thy seed after thee, which shall be of thy sons; and I will establish his kingdom. 12 He shall build me an house, and I will stablish his throne for ever. 13 I will be his father, and he shall be my son: and I will not take my mercy away from him, as I took it from him that was before thee: 14 But I will settle him in mine house and in my kingdom for ever: and his throne shall be established for evermore.
8. That the redeemer would be a king to sit upon the throne of David.
Isaiah 9:7 Of the increase of his government and peace there shall be no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even for ever. The zeal of the LORD of hosts will perform this.
9. That the redeemer will establish the kingdom of God on this earth.
Isaiah 49:6 And he said, It is a light thing that thou shouldest be my servant to raise up the tribes of Jacob, and to restore the preserved of Israel: I will also give thee for a light to the Gentiles, that thou mayest be my salvation unto the end of the earth.
Psalm 2:7 I will declare the decree: the LORD hath said unto me, Thou art my Son; this day have I begotten thee. 8 Ask of me, and I shall give thee the heathen for thine inheritance, and the uttermost parts of the earth for thy possession. 9 Thou shalt break them with a rod of iron; thou shalt dash them in pieces like a potter’s vessel. 10 Be wise now therefore, O ye kings: be instructed, ye judges of the earth. 11 Serve the LORD with fear, and rejoice with trembling. 12 Kiss the Son, lest he be angry, and ye perish from the way, when his wrath is kindled but a little. Blessed are all they that put their trust in him.
10. Prophecies of the future right through to the kingdom of God.
Amos 3:7 Surely the Lord GOD will do nothing, but he revealeth his secret unto his servants the prophets.
Daniel 2:44 And in the days of these kings shall the God of heaven set up a kingdom, which shall never be destroyed: and the kingdom shall not be left to other people, but it shall break in pieces and consume all these kingdoms, and it shall stand for ever.
11. How Israel failed to keep the Covenant, so God promised a New Covenant (the New Testament).
Jeremiah 31:31 Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah: 32 Not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they brake, although I was an husband unto them, saith the LORD: 33 But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people. 34 And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more.
12. A future resurrection and judgment.
Daniel 12:2 And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.
The importance of the Old Testament cannot be overstated.
Luke 24:25 Then he said unto them, O fools, and slow of heart to believe all that the prophets have spoken: 26 Ought not Christ to have suffered these things, and to enter into his glory? 27 And beginning at Moses and all the prophets, he expounded unto them in all the scriptures the things concerning himself.
John 5:39 Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me.
John 5:46 For had ye believed Moses, ye would have believed me: for he wrote of me.
I hope you find this helpful.