The passage from II Timothy 3:16 affirms that all Scripture is from God. As a follower of Jesus the Christ it must go deeper. By that I mean it is not enough to state that one believes the Scriptures are inspired by God. We must come to the deeper point of knowing that what God says is right (Romans 3:4). As Christians we are to sanctify the Lord God in our hearts in order to answer when someone asks us about God (I Peter 3:15). Belief in God means that we also live for God. We are to love God with all our heart, soul and mind (Matthew 22:37). Jesus spoke of those whose heart was far from Him (Matthew 15:8). Trusting what God has said should come from a heart which has learned that God knows best.
By way of example, here is a man and woman living together without being married. They may say they believe the Bible is from God but is God right in telling them to obey the laws of the land (Romans 13:1)? The smart answer given by the world is, marriage is more than just a piece of paper. Who says otherwise? However, that piece of paper or whatever it takes to comply with the laws of God comes from God. Here is a person who wins a $1000 by playing the lottery. Where did the money come from? Before one says the state or city government let us go back a little further. That $1000 came from people who give up their grocery or bill money to hit it rich and solve all their problems. Is God right to say, if one does not work they do not eat? (II Thessalonians 3:10)
The growth toward maturity of the spiritual man is realizing that the commands of God are not grievous (I John 5:3). One finds joy in worshipping God and not seeing it as drudgery (Malachi 1:13). He finds joy in being around people who believe like he does and hence will not forsake assembling with other Christians (Hebrews 10:25). He gives his money from a cheerful heart because he sees it as a gift to God (II Corinthians 9:7). If we are looking for a short quick answer then men do not trust what God has said because they do not understand what God is doing for them in the commands. The abundance of life which Jesus offers to those on earth is brought about by trusting that God knows what is ultimately best for all mankind, including me.