Why should prophecy make us live holy lives?
Prophecy is meant to make us holy because our righteous, consecrated life is itself the testimony of Jesus, which Revelation 19:10 calls the spirit of prophecy; when we live holy, we fulfill the eternal purpose God spoke over us before the world began and let the world see Jesus walking the earth again through us.
Scripture teaches that the testimony of Jesus Christ is the spirit of prophecy. Revelation 19:10 says the servant told John, "worship God: for the testimony of Jesus is the spirit of prophecy." This means our holy life is not simply a moral requirement, it is the very fulfillment of what God spoke about us in eternity past. Titus 1:1-3 shows that God, who cannot lie, promised eternal life before the world began, and that promise has now been manifested through preaching and through the life of the believer. So when we walk in holiness, we are living out a prophecy that was declared over us before time began.
Because the testimony of Jesus Christ is the Spirit or Life of Christ manifested in and through us, it is by our righteous, chaste life that the world is able to see Jesus walk the earth again. It is by the holiness and consecrated life of believers that the spirit of prophecy is actually manifested in the earth. Everyone who carries the testimony of Jesus Christ is a spirit of prophecy walking around, whether or not they hold the title of prophet. This is why holy living matters so much: it is the visible, ongoing proof that Christ's testimony and God's eternal counsel are being fulfilled through us.
Holds also that holiness is the evidence of Christ in us, and that without it God will not manifest Himself in or through a life or ministry. If we are not holy, we cannot properly discern sin or walk in the standard God requires, and the power of Christ that the world is truly searching for cannot be revealed through us. Purity is described as essential to the whole body, because impurity in even one member affects the effectiveness of the whole. In short, prophecy calls us to holiness because holy living is the very way the eternal word and testimony of Jesus becomes visible and active in the world today.
The scriptures
And I fell at his feet to worship him. And he said unto me, See thou do it not: I am thy fellowservant, and of thy brethren that have the testimony of Jesus: worship God: for the testimony of Jesus is the spirit of prophecy. And I saw heaven opened, and behold a white horse; and he that sat upon him was called Faithful and True, and in righteousness he doth judge and make war. His eyes were as a flame of fire, and on his head were many crowns; and he had a name written, that no man knew, but he himself. And he was clothed with a vesture dipped in blood: and his name is called The Word of God.
Paul, a servant of God, and an apostle of Jesus Christ, according to the faith of God’s elect, and the acknowledging of the truth which is after godliness; In hope of eternal life, which God, that cannot lie, promised before the world began; But hath in due times manifested his word through preaching, which is committed unto me according to the commandment of God our Saviour;
If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son. He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son. And this is the record, that God hath given to us eternal life, and this life is in his Son.
For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them.
And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ: To the intent that now unto the principalities and powers in heavenly places might be known by the church the manifold wisdom of God,
And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.
He that hath my commandments, and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him, and will manifest myself to him.
A little leaven leaveneth the whole lump.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.