Many who go/live/walk only by the New Testament Scriptures err in the Scriptures and have no understanding of God.
God is clear in His two witnesses: the Scriptures, both Old and New Testaments. They testify of Him. The Old Testament Scriptures are the foundation on which the New Testament is written.
Jesus was so very clear when He was referencing the writings of Moses. If we do not believe them, we will not believe Jesus, for Moses wrote of Him. He said search the Scriptures. They testify of Him. God tells us His word is to be understood line upon line, line upon line; precept upon precept, precept upon precept; here a little and there a little. All Scripture is given by the inspiration of God. And it is profitable for doctrine, for reproof, for correction, for instruction in righteousness. (Read John 5:39; Isaiah 28:9,10; 2 Timothy 3:16.)
We are not to reason using as a basis our own opinions or perceptions, nor the opinions or perceptions of others. Only by the Word of God is it safe to reason. It is thus that we can be led of the Holy Spirit and gain understanding. Trusting the Spirit of God, we can be guided into all truth. (Read John 16:13.)
“There is a way which seemeth right unto a man, but the end thereof are the ways of death” (Proverbs 14:12).
We are to live by God’s Holy Word. The Old and New Testaments, God’s law, God’s Commandments are His light to guide our paths. There is in God’s word counsel, instruction in righteousness for every life circumstance and situation. Period. God, through King Solomon, said there is nothing new under the sun. That which was, is, and will be again. (Read Ecclesiastes 1:9, 10.) Let that marinate.