"The testimony of Jesus is the spirit of prophecy"
is a way of saying that Jesus Christ is the theme of the Bible from beginning to end. Every book in the Old Testament/Tanakh pointed to Jesus prophetically, not just in the books we label "prophetic" (i.e., Isaiah through Malachi) but also in each and every other book, whether historical, poetic, or legislative ("Law/Torah").
From Genesis to Malachi (or in the Tanakh, from Genesis to Chronicles) there is written testimony that bears witness to the centrality of Jesus in the grand arc of history, from the creation of the angels, some of whom were cast out of heaven, to the creation of the new heavens and the new earth in which righteousness will dwell forever. All of history is bound up in the unfolding story of the redemption of humankind which was accomplished once and for all with the perfect life and the death, burial, and resurrection of Jesus Christ, who from the cross cried out in a loud voice
"Finished!" (Matthew 27:50; John 19:30).
What was finished at the cross began, of course, in the eternal counsels of the Godhead, before the foundations of the world and even before the creation of the angelic hosts (see, e.g., Ephesians 1:7-11; 1 Peter 1:18-20). From eternity God had the "big picture" of what He intended to do with the sentient beings He would bring into existence, first in heaven and then on planet Earth. His overarching purpose for them, celestial and terrestrial, was for them--in the words of one of the Confessions of the church--"to glorify God and to enjoy Him forever."
We know what happened next, first in heaven and then on earth, when perhaps a third of the heavenly hosts followed Lucifer, "son of the morning," in his rebellion against God Almighty, and when our first parents disobeyed God and by extension took the entire human race with them in their rebellion (since "in Adam, all die"--1 Corinthians 15:22).
God, in the opening chapters of the book of beginnings (Genesis/Bereishis), in what is called the "Proto Evangelium," or the "first gospel," revealed the first prophetic word which gave humankind the first inkling as to what God's plan for the ages would entail.
Before we consider this prophetic word, we must consider the events leading up to its being given to our first parents:
Adam and Eve were created in the image and likeness of the invisible God, with Adam being created first and given the headship of the created order of living things God had placed in the earth and in the air and the sea, and Eve being created second as both a fit helper to and equal partner of the man. God then commanded them to bear children and thus fill and subdue the entire earth and exercise dominion over all that is in the earth.
Adam forfeited his rulership, or delegated dominion (Genesis 1:27), over God's earthly creation by allowing his wife Eve to partake of the only fruit (of perhaps hundreds or even thousands) of which God had expressly forbidden them to partake. Had Adam taken his role of dominion seriously and had 1.) forbidden his helpmeet, Eve, from partaking of the forbidden fruit, and 2.) told the serpent to "beat it," history would have been written differently. We know, however, the scenario did not unfold in that way. Adam, who was with his wife at the time she was deceived by the tempter (Genesis 3:6) allowed her to eat the forbidden fruit and then ate it himself.
Adam and his wife, rather than admitting their disobedience to God, make excuses--what today we would call rationalizations, with Adam blaming Eve, and Eve blaming the serpent (3:12,13).
God said to the tempter, "And I will put enmity between you and the woman, and between your seed and her seed; He shall bruise you on the head, and you shall bruise Him on the heel" (3:15).
In other words, since Adam had failed to exercise dominion as God intended, God Himself would provide through the seed of the woman "the last Adam"--that is, the perfect God-Man, Jesus, who would wrest from Satan the dominion that Adam had forfeited to him through disobedience. Jesus would thereby become to all humankind "a life-giving spirit" (see 1 Corinthians 15:45-49).
In so doing, Jesus' efforts to that end would result in the heel of His foot being bruised by the seed of the serpent, which comprised both the demonic spirits who joined Satan in his rebellion in heaven and unbelieving humanity who followed Satan's lead on the earth. Praise be to God, however, the very foot that was bruised at Golgotha (Calvary) would crush the poison-filled head of the serpent at the cross of Christ, thus delivering a death blow to Satan.
By having lost his dominion as the "god of this world" Satan played right into God's hands, so that God, in a holy, righteous, and just way could restore to humanity all they had lost in the fall. Before Jesus could become a life-giving spirit to each individual member of humankind, however, each person would need first to repent, and second to believe in the seed of the woman; that is, Jesus. As Paul reminds us in Galatians,
"But when the fullness of the time came, God sent forth His Son, born of a woman, born under the Law, so that He might redeem those who were under the Law, that we might receive the adoption as sons" (4:4:5).
And as John tells us in the first chapter of his gospel,
". . . as many as received [Jesus], to them He gave the right to become children of God, even to those who believe in His name" (v.12).
In conclusion, I refer you to an excellent chart which lays out for us very nicely the ways in which the testimony of Jesus can be found in every book of the Bible. It is here: http://www.jesusplusnothing.com/jesus66books.htm. I commend it to you. Going through the chart from beginning to end will likely cement in your mind and heart how the rich tapestry of scripture, though comprising many skeins, has one particularly beautiful skein the color of blood.
That skein had its beginning in Genesis 3:21 where God clothed His erring creatures in the Garden of Eden with the skins of animals whose blood had been shed. It has its ending in the Revelation of Jesus Christ where we read that
"[Jesus] is clothed with a robe dipped in blood, and His name is called 'The Word of God'" (19:13).
The prophetic word in Genesis 3:15 presaged the blood of the Lamb, who through His death bore our sins in His own body on the tree (1 Peter 2:24), and that word was fulfilled to the letter by the Logos, the living Word of God, who is our Lord and Savior, Jesus Christ.
Truly, the testimony of Jesus is the spirit of prophecy, and each book in the Bible points to Him! (See, for example, Psalm 22 and Isaiah 53.)