Of course it isn't written to unbelievers. The Bible is not the power of God unto salvation to them that believe, the preaching of the Gospel of the free and sovereign grace of God in Christ Jesus the Lord is.
Romans 1:16 (KJV) For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
1 Corinthians 1:18 (KJV) For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God.
Romans 10:13-15 (KJV) 13 For whosoever shall call upon the name of the Lord shall be saved. 14 How then shall they call on him in whom they have not believed? and how shall they believe in him of whom they have not heard? and how shall they hear without a preacher? 15 And how shall they preach, except they be sent? as it is written, How beautiful are the feet of them that preach the gospel of peace, and bring glad tidings of good things!
There is a huge difference between reading the Bible and understanding the Bible. Anyone can read the words and grasp the sentences but it takes the Spirit to open up the Word. Those who do not have the Spirit cannot understand the Scriptures.
1 Corinthians 2:10-16 (KJV) 10 But God hath revealed [them] unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God. 11 For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God. 12 Now we have received, not the spirit of the world, but the spirit which is of God; that we might know the things that are freely given to us of God. 13 Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual. 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. 15 But he that is spiritual judgeth all things, yet he himself is judged of no man. 16 For who hath known the mind of the Lord, that he may instruct him? But we have the mind of Christ.
Oh, so you are one of those who don't believe a person can read the Word of God and be saved, but must have some lofty theological training?
The Bible repeatedly claims to be God's word. See 1 Corinthians 14:37; 2:10-13; Ephesians 3:3-5; John 16:13; Matthew 10:19,20; Galatians 1:8-12; 2 Peter 1:20,21; 1 Thessalonians 2:13; 2 Timothy 3:16,17; Luke 10:16.
If the Bible is God's word, it must harmonize with God's nature. When we study the Bible and when we understand God's nature, then we necessarily must believe that we can understand the Bible. To claim that we cannot understand the Bible is either to deny that it is God's word or to impugn the character of God Himself.
1 Timothy 2:4 - God desires all men to be saved and to come to the knowledge of the truth. God does not want some to understand, but others are incapable. He wants all to know the truth.
2 Timothy 3:15-17 - The Scriptures were revealed by God to teach and instruct us in righteousness and provide us to all good works.
Romans 1:16 - The gospel of Christ is the power of God to salvation for everyone who believes.
Titus 1:1-3 - God manifested (make known) His word in the message which was entrusted to Paul according to God's command.
The very words in the Bible were chosen by the all-wise, all-powerful God.
1 Corinthians 2:12,13 - Inspired men received God's Spirit so they could know what God revealed and could speak them in words which the Holy Spirit taught them.
God commands and requires men to understand His word.
Mark 7:14 - Jesus' teachings were intended for the common people (multitudes), and He expected and commanded them all to understand His teachings.
Luke 8:11; Matthew 13:19,23 - The seed of the gospel is the word of God. Those whom the Lord considers to be good and fruitful are those who understand the word. Those who do not understand are unacceptable.
So God requires and commands all people to understand, and we must understand to have His approval and to have faith. Yet how can God be just, loving, and gracious in these requirements, if in fact understanding is impossible?
[Colossians 1:9-11; Philippians 1:9-11; James 1:21-25]
God will judge men and reward them eternally according to their obedience to His word.
John 12:48 - Jesus said, "The word that I have spoken will judge him in the last day." [Romans 2:16]
Acts 17:30,31 - These times of ignorance God overlooked, but now commands all men everywhere to repent, because He has appointed a day on which He will judge the world by the Man whom He has ordained.
God always speaks truth and always keeps His promises. But in the following passages He has stated or promised that we can understand His word.
2 Timothy 3:15-17 - The Scriptures are able to make us wise to salvation. They are profitable to teach and instruct us and to provide us to all good works. How could they make us wise and profit us in these things if we cannot understand them?
Isaiah 55:11 - God's word will not return to Him void, but it shall accomplish what He pleases and prosper in the thing for which He sent it. But He sent it to reveal His will to men, etc. If we cannot understand it, then it does not prosper nor did it accomplish what He sent it for.