A person cannot be saved without knowing God’s will (Matthew 7:21). The will of Christ came into effect following His death on the cross and His resurrection into heaven. His will became active beginning in Acts 2. God expects me to search all of the scriptures to arrive at the right answers for life. Paul addressed the matter of things in the Old Testament by saying they were written for our learning (Romans 15:4). Those in Berea searched the scriptures daily to verify that what they had heard was correct (Acts 17:11). All that was available at that time was the Old Testament. The things that happened to Israel (Old Testament) were an example for us (I Corinthians 10:11).
We are saved by obeying the truth (I Peter 1:22). I use all sixty-six books of the Bible to arrive at truth. The will of Christ could not come into effect until after He died (Hebrews 9:16,17). The thief on the cross was dead and buried some fifty days before the will of Christ came into action. God reaches our hearts (that is our thinking) by means of truth. Each individual must make every effort possible to understand God’s will for their life. I read the Scriptures, consider commentaries, read books on given issues and, at times, ask people in whom I have confidence, what does this mean?
Material outside of the written Word as cited above may help us to guide our thinking in the right direction. In the end, however, I must decide what God is saying to me. In light of II Timothy 2:15, it is not subjective thinking. By that I can reach no conclusions other than those demanded by the evidence of scripture. The eunuch did not understand what he was reading and asked for help (Acts 8:31). Supposing that Philip had told the eunuch that this passage refers to Mohammed? The evidence of scripture would show such a statement to be false. The eunuch reached the right conclusion by accepting the evidence. He reached the conclusion that Jesus is the Christ the Son of God. Do not trust any man or book above what is written (I Corinthians 4:6).