Both the Old and New Testaments are the WORD of GOD because they are composed of scriptures which were given by inspiration of God.
2 Timothy 3:16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: 17 That the man of God may be perfect, throughly furnished unto all good works.
The words of Christ are the words of God.
John 3:34 For he whom God hath sent speaketh the words of God: for God giveth not the Spirit by measure unto him.
John 12:49 For I have not spoken of myself; but the Father which sent me, he gave me a commandment, what I should say, and what I should speak.
John 14:24 He that loveth me not keepeth not my sayings: and the word which ye hear is not mine, but the Father’s which sent me.
The Old Testament is largely concerned with the Old Covenant which God made with Israel at Sinai.
This Covenant contained laws for the governance of the nation.
Israel failed to keep the Covenant and God promised a New Covenant.
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.
The Old Covenant served to convict men and women of sin for which the penalty is death.
Romans 3:20 Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
1 John 3:4… sin is the transgression of the law.
Ezekiel 18:20 The soul that sinneth, it shall die.
Romans 6:23 For the wages of sin is death;
The New Testament is concerned with the New Covenant whereby sin can be forgiven, and the sentence of death annulled.
The writer to the Hebrews explains:
Hebrews 8:8 For finding fault with them, he saith, Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel and with the house of Judah:
The New Covenant replaces the Old
Hebrews 8:13 In that he saith, A new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away.
Jesus Christ is the mediator of the New Covenant.
Hebrews 12:24 And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than that of Abel.
Jesus Christ is the messiah (anointed one) foretold in the Old Testament who made the forgiveness of sin possible by giving his sinless life as a sacrifice.
Hebrews 10:12 But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God;
The New Covenant requires understanding and belief of the gospel of the kingdom of God preached by Jesus and his apostles, followed by repentance and baptism by immersion in water into Christ for the remission of past sins.
After baptism we must live a new life by the commandments of Christ.
Romans 6:4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
Jesus Christ fulfilled the Old Covenant and the Law of Moses and instituted the New Covenant and the Law of Christ.
Galatians 6:2 Bear ye one another’s burdens, and so fulfil the law of Christ.
James 1:22 But be ye doers of the word, and not hearers only, deceiving your own selves.
The New Covenant and the Law of Christ are for the governance of the individual in any nation.
They cannot be used for the governance of a nation.
We cannot live a sinless life but if we obey the gospel, we are promised forgiveness of sin because of our belief (faith) in Christ.
Galatians 3:22 But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe.
1 John 1:7 But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin. 8 If we say that we have no sin, we deceive ourselves, and the truth is not in us. 9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
When Christ returns to judge the world in righteousness the faithful will be raised from the dead to eternal life in the kingdom of God on earth.
We demonstrate our faith by the way that we live.
Matthew 5:16 Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven.
Salvation for those who lived before Christ came depends upon their faith in the promise of God to provide a redeemer of which there is ample testimony in the Old Testament.
Salvation for those who live after Christ depends on their belief of the gospel and their faith in him as the promised redeemer.
Romans 1:17 For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith.
Romans 3:30 Seeing it is one God, which shall justify the circumcision by faith, and uncircumcision through faith.
I hope this helps