Why must all prophetic scripture be fulfilled?
All prophetic scripture must be fulfilled because God's truthfulness is at stake. If even the smallest part failed, God would be a liar. Everything written prophetically must come to pass, including Christ's suffering, which Peter says God fulfilled by the mouths of all His prophets.
Les teaches that anytime something is written in prophecy, it has to be fulfilled to the last jot and tittle, or it would make God a liar. He stresses that everything written prophetically must come to pass, and everything still future will happen; otherwise God would be a liar. In Acts chapter 3, Peter acknowledges that the rulers acted in ignorance when they crucified the Lord, but the flip side is that God had before shown by the mouth of all His prophets that Christ should suffer, and He hath fulfilled it. So Christ had to die because prophecy had foretold it. Peter then calls them to repent and be converted, changing their thinking and direction.