The Law. Written by Moses (a believer) to and for Covenant Israel (other believers).
The Psalms. Written by Solomon, David, etc. (believers) to and for Covenant Israel (other believers)
The Prophets. Written by Ezra, Ruth, Samuel, Ezekiel, Daniel, Malachi, etc (believers) to and for Covenant Israel (other believers).
The Gospels. Written by Matthew, Mark, Luke, John (believers) to and for the Church (other believers).
The Acts of the Holy Spirit. Written by Luke the Physician (believer) to and for the Church (other believers).
The Epistles. Written by Paul, Peter, James, John, etc. (believers) to and for the Church (other believers).
The Revelation of Jesus Christ. Written by John (believer) to and for the Church (other believers).
Jesus Himself speaks to His disciples in response to the question of why He speaks to the people in parables:
11 And he said unto them, Unto you it is given to know the mystery of the kingdom of God: but unto them that are without, all these things are done in parables:
12 That seeing they may see, and not perceive; and hearing they may hear, and not understand; lest at any time they should be converted, and their sins should be forgiven them.
Jesus, in speaking parables presents some aspect or teaching of the Gospel of the kingdom of God to the people and also instructs, commands, and invites them to repent and be saved, and actively prevents the people from seeing, hearing, and understanding, and also to prevent them from repenting and being converted. The reason for this is simple: the people are not worthy of the salvation and the mercy of the grace of God.
Christ, in another place says:
63 It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.
The Word of God is spirit and life. The unconverted are flesh and death. Both, as Paul described and taught, are contrary one to another:
17 For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
Paul says elsewhere:
1 Corinthians 2:14
14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.
Man is flesh, the Word of God is spiritual. Man cannot "receive" the things of the Spirit of God "neither can he know them" because of this judgment upon them of God. It is only through God converting the individual and putting His Spirit in them does a man become a spirtual man and then the Word of God can be heard, can be seen, can be understood, and in this the Word of God applies to the believer in Covenant with God.
The Word of God is for believers ONLY.