To begin with, the apostle Paul ( see 2 Timothy 1:1 ) wrote ( see 1:3 and 2:7, for example ) in 3:16 that all Scripture is breathed out by God.  The source of the Holy Bible is not man, but God.  As such, it is absolutely authoritative.

Secondly, Paul wrote that all Scripture is profitable for teaching, for example.  In Matthew 6:17 ( New Testament; The [ expanded ] Bible™; Study The Bible While You Read ), it is written, " ¹⁷So when you fast [v. 16], ·comb your hair [Lput oil on/anoint your head; ^Ctypical first century grooming] and wash your face. "  Fathers have the primary responsibility for teaching this, for example, for in Ephesians 6:4, it is written, " ⁴ Fathers, do not provoke your children to anger, ^u but bring them up in the discipline and instruction of the Lord. "  God commands ( see Ecclesiastes 12:13, for example ) that children be taught godly grooming habits, for example.  See also Proverbs 1:8, for example.

Thirdly, all Scripture is profitable for reproof, for example.  One purpose of God's word is to convict us of our sinfulness,¹ as we find in the definition of the Greek word ( Greek [ 1650 ] ) for the word " reproof, " in 2 Timothy 3:16, in the King James Version ( KJV ) of the Holy Bible.  Implied in God's command to examine the Scriptures daily ( see Acts 17:10, 11, for example ) is that we will be convicted of our sinfulness on a daily basis.

Finally, we need the Bible so that we can be trained in righteousness.  God expects us to grow in the grace and knowledge of our Lord and Savior Jesus Christ ( see 2 Peter 3:18, for example ).  All Scripture is breathed out by God, so that we can be assured of living Christian lives, inspired by the Master in heaven ( see Matthew 18:35 and Romans, chapter 6 ( especially verses 3, 4, and 22, for example ), for example ).