Why Tanakh (including "[Brit] Hadashah") is the Word of G-d

1. "Ye shall not add unto the word which I command you, neither shall ye diminish from it, that ye may keep the commandments of the LORD your God which I command you." (Deuteronomy 4:2, JPS)
2. "All this word which I command you, that shall ye observe to do; thou shalt not add thereto, nor diminish from it." (Deuteronomy 13:1)
3. "Every word of God is tried; He is a shield unto them that take refuge in Him. Add thou not unto His words, lest He reprove thee, and thou be found a liar." (Proverbs 30:5-6)

I could give other parshot (readings, verses); but you get the point. Meanwhile:

14 Now therefore fear the LORD, and serve Him in sincerity and in truth; and put away the gods which your fathers served beyond the River, and in Egypt; and serve ye the LORD.

15 And if it seem evil unto you to serve the LORD, choose you this day whom ye will serve; whether the gods which your fathers served that were beyond the River, or the gods of the Amorites, in whose land ye dwell; but as for me and my house, we will serve the LORD.' {P}

(Y'hoshu'a 24, JPS)