The Main Way God Speaks —The advantage of written revelation
Throughout history God has "at various times and in various ways spoken in times past by the prophets..." (Hebrews 1:1). However, it has been relatively rare for God to speak by the mouths of his prophets. The main form and method by which God has provided a revelation of his will is the written word. The revelation from God in the Bible may be second hand, but it is by no means second rate.
1 The Dearth of Oral Revelation
In Samuel’s time the "word from the Lord was rare"(1Samuel 3:1).
Much of Israel’s history likewise saw "a famine... not for bread or a thirst for water, but rather for hearing the word of the Lord"(Amos 8:11).
Jesus spoke to the multitudes but they were very few compared to the uncountable host who have "heard" him through the written records of Matthew, Mark, Luke, and John (Matthew 5:1,Matthew 7:28).
We live in a time of "famine" regarding oral messages from the Lord, yet we have a feast spread before us in the sacred writings preserved for us. Paul said, "All scripture is given by inspiration of God and is profitable... that the man of God may be complete, thoroughly equipped for every good work" (2Timothy 3:16).
2 The Dependence of Oral Revelation on Scripture
When Peter preached on Pentecost he did not supersede the scriptures with his words, rather he used and explained the scriptures, and made them the source of his message (Acts 2:14-36).
The Bereans listened to the inspired spoken word. Then they searched the scriptures to verify what was spoken, and were the "more noble" for doing so (Acts 17:10-12).
Timothy in a time of miracles relied on the scriptures in his ministry as an evangelist (1Timothy 3:15,1Timothy 4:13,2Timothy 3:14-16).
The apostles even in writing scripture used scripture. For example Paul in one passage quotes from both old and new testament writings (1Timothy 5:17-18,Deuteronomy 25:4,Luke 10:7).
3 The Superiority of Written Revelation
Oral prophecy in Paul’s day was only "in part"(1Corinthians 13:9-12) and we should compare that statement with Paul’s statement that the scriptures completely equip the people of God. That which is in part is obviously not as good that which is perfect and which makes us complete (2Timothy 3:16).
Even when oral prophets could be consulted God expected his written word to be heeded. For example Huldah’s prophecy was subservient to the book of the law, not a substitute for it (2Chronicles 34:20-24).
When the rich man was dead and in torment, he begged Abraham to send Lazarus in person to the rich man’s brothers. But Abraham said, "They have Moses and the prophets, let them hear them"(Luke 16:19-31). They did not have Moses and the prophets in person, but in the Old Testament scriptures. The rich man thought an oral and personal message would be better, but he was wrong.
The oral revelations would fail and vanish away, for they were a piecemeal and transitory manner of revelation. That's why the words of Jesus, for example, were written in books. It is the written teaching that has lasted and come down to us (1Corinthians 13:8-13).